UNIVERSITY  OF  CALIFORNIA. 


GIFT    OF 


Class 


3  to 


SPECTRUM  ANALYSIS 

EXPLAINED. 


ITS    USES    TO     SCIENCE    ILLUSTRATED,     SHOWING     ITS     APPLICATION 

IN    MICROSCOPICAL  RESEARCH,  AND    TO    DISCOVERIES  OF  THE 

PHYSICAL     CONDITION    AND     MOVEMENTS     OF    THE 

HEAVENLY     BODIES,    AND     INCLUDING    AN 

EXPLANATION  OF  THE  RECEIVED 

THEORY    OF 

SOUND,  HEAT,   LIGHT,   AND   COLOR. 


COMPILED    BY   THE   EDITOR   OF 

"  HALF-HOUR    RECREATIONS    IN   POPULAR    SCIENCE," 

FROM    THE   WORKS    AND    OB   ERVATIONS    OF 

PROFS.  SCHELLEN,  ROSCOE,  HUGGINS,  LOCKYER,  YOUNG, 
AND  OTHERS. 


h  5  is 


FULLY 


BOSTON : 

ESTES     AND     LAURIAT, 
143  WASHINGTON  STREET. 

1872. 


Entered,  according  to  Act  of  Congress,  in  the  year  1872, 

BY  DANA  ESTES, 
In  the  Office  of  the  Librarian  of  Congress,  at  Washington. 


Stereotyped  at  the  Boston  Stereotype  Foundry, 
19  Spring  Lane. 


SCHELLEN.]       SPECTRUM  ANALYSIS  EXPLAINED.  55 


4.    Spectrum  Analysis  Explained,  and  its  llses 
to  Science  Illustrated* 

INTRODUCTORY. 

THIS  work  is  founded  upon  a  series  of  lectures  deliv- 
ered by  the  author  during  the  winter  of  1869,  before 
the  "  Society  for  Scientific  Lectures,"  in  Cologne.  Its 
object  is,  on  the  one  hand,  to  give  a  clear  and  familiar 
representation  of  the  nature  and  phenomena  of  spectrum 
analysis,  enabling  an  educated  person  not  previously  familiar 
with  physical  science  to  become  acquainted  with  the  newest 
and  most  brilliant  discovery  of  this  century ;  and,  on  the 
other  hand,  to  show  the  important  position  which  spectrum 
analysis  has  acquired  in  the  pursuit  of  physics,  chemistry, 
technology,  physiology,  and  astronomy,  as  well  as  its  adapta- 
bility to  almost  every  kind  of  scientific  investigation. 

The  general  reader  will  be  introduced  by  this  book  into  a 
new  realm  of  science,  the  dominion  of  which  has  extended 
in  a  few  years  over  all  terrestrial  substances,  and  even  be- 
yond them  to  the  most  distant  parts  of  the  universe.  He 
will  learn  to  decipher  the  new  language  of  light,  which  by 
unequivocal  signs  yields  him  information  not  only  concerning 
the  nature  of  terrestrial  substances,  but  also  of  the  physical 
constitution  of  the  heavenly  bodies. 

To  facilitate  the  due  appreciation  of  the  results  which 
have  been  obtained  by  the  application  of  spectrum  analysis 
to  the  heavenly  bodies,  the  author  has  given  with  each  class 
of  objects  a  summary  of  the  information  hitherto  furnished 
by  the  telescope,  and  has  sought  to  give  a  glance  in  passing 
at  the  progressive  development  and  partial  transformation 
of  the  heavenly  bodies. 

The  author  acknowledges  with  grateful  thanks  the  val- 
uable assistance  rendered  him  by  various  scientific  men  who 

I 

237471 


56  RECREATIONS  IN  POPULAR  SCIENCE.     [SCHELLEN. 

have  kindly  communicated  to  him  the  results  of  their  labors, 
among  whom  he  would  especially  mention  Messrs.  Huggins, 
Secchi,  Lockyer,  Zollner,  Janssen,  Morton,  and  Young. 


ON  THE  ARTIFICIAL  SOURCES  OF  HIGH  DEGREES  OF  HEAT 
AND  LIGHT. 

The  total  eclipse  of  the  sun  in  India  of  the  i8th  of  Au- 
gust, 1868,  was  an  event  which,  it  will  be  remembered, 
excited  extreme  interest  in  the  scientific  world,  and  led  to  a 
large  expenditure  of  money  and  labor,  in  order  that  a  new 
method  of  investigation —  spectrum  analysis  —  might  be  ap- 
plied to  those  mysterious  phenomena  invariably  present  at  a 
total  solar  eclipse,  the  nature  and  character  of  which  the 
unassisted  powers  of  the  telescope  had  proved  themselves 
inadequate  to  reveal.  The  brilliant  results  obtained  at  this 
eclipse  were  fully  confirmed  by  the  more  recent  observations 
made  in  North  America  during  the  total  eclipse  of  the  7th 
of  August,  1869,  and  the  records  of  those  eclipses  laid  be- 
fore the  various  scientific  societies  clearly  assert  the  triumph 
of  spectrum  analysis.  On  this  account,  the  new  method 
of  investigation  has  excited  great  interest  in  all  cultivated 
circles,  and  therefore  a  familiar  and  comprehensive  exposi- 
tion of  the  details  of  spectrum  analysis,  in  which  is  shown 
the  great  value  of  this  method  of  research  in  every  depart- 
ment of  physical  science,  seems  not  uncalled  for. 

By  spectrum  is  not  understood  in  physics  a  spectre  or 
ghostly  apparition,  as  the  verbal  interpretation  of  the  word 
might  well  lead  one  to  suppose,  but  that  beautiful  image, 
brilliant  with  all  the  colors  of  the  rainbow,  which  is  obtained 
when  the  light  of  the  sun,  or  any  other  brilliant  object,  is 
allowed  to  pass  through  a  triangular  piece  of  glass  —  a 
prism. 

The  unassisted  eye  can  perceive  no  difference  in  the  light 
from  the  heavenly  bodies  and  that  from  various  artificial 
sources,  beyond  a  variation  in  color  and  brilliancy  ;  but  it  is 

2 


SCHELLEN.]       SPECTRUM  ANALYSIS  EXPLAINED.  57 

quite  otherwise  when  the  light  is  viewed  through  a  prism. 
There  are  then  formed  very  beautiful  colored  images  or 
spectra,  the  constitution  and  appearance  of  which  depend 
upon  the  nature  of  the  substance  emitting  the  light.  The 
different  appearances  presented  by  these  colored  images  are 
so  entirely  characteristic,  that  to  every  substance,  when 
luminous  in  a  gaseous  form,  there  corresponds  a  peculiar 
spectrum,  which  belongs  only  to  that  particular  substance. 

It  follows,  therefore,  that  when  the  spectra  of  different 
substances  have  been  determined  once  for  all,  by  previous 
researches,  and  have  been  recorded  in  maps  or  impressed 
upon  the  memory,  it  is  easy  in  any  future  investigation  to 
recognize  at  once,  from  the  form  of  the  spectrum  which  a 
body  of  unknown  constitution  presents,  the  individual  sub- 
stances of  which  it  is  composed. 

This  statement  presents  in  general  terms  the  nature  of 
spectrum  analysis.  It  analyzes  bodies  into  their  constit- 
uent parts,  not  as  the  chemist,  with  alembics  and  retorts, 
with  re-agents  and  precipitates,  but  by  means  of  the  spectra 
which  these  substances  give  when  in  a  state  of  intense 
luminosity. 

Spectrum  analysis  in  no  way  supplants  the  methods  of 
chemical  analysis  hitherto  in  use ;  for  its  function  is  neither 
to  decompose  nor  to  combine  bodies,  but  rather  to  reconnoitre 
an  unknown  territory,  and  to  stand  sentinel,  and  signalize  to 
the  physicist,  the  chemist,  and  the  astronomer,  the  presence 
of  any  substance  brought  beneath  its  scrutiny. 

With  what  acuteness,  with  what  delicacy  does  spectrum 
analysis  accomplish  this  task !  When  the  balance,  the  mi- 
croscope, and  every  other  means  of  research  at  the  command 
of  the  physicist  and  the  chemist  utterly  fail,  one  look  in  the 
spectroscope  is  sufficient,  in  most  cases,  to  reveal  the  presence 
of  a  substance.  If  a  pound  of  common  salt  be  divided  into 
500,000  equal  parts,  the  weight  of  one  of  these  portions  is 
called  a  milligramme.  The  chemist  is  able,  by  the  use  of 
the  most  delicate  scales  and  the  application  of  special  skill, 

3 


58  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLEN. 

to  determine  the  weight  of  such  a  particle  ;  but  in  doing  so, 
he  comes  close  upon  the  limits  of  his  power  of  detecting  by 
chemical  means  the  presence  of  sodium,  the  chief  element 
in  common  salt.  But  if  that  small  milligramme  be  subdi- 
vided into  three  million  parts,  we  arrive  at  so  minute  a  par- 
ticle that  all  power  of  discerning  it  fails,  and  yet  even  this 
excessively  small  quantity  is  sufficient  to  be  recognized  with 
certainty  in  a  spectroscope.  We  have  but  to  strike  together 
the  pages  of  an  old  dusty  book  in  order  to  perceive  immedi- 
ately in  a  spectroscope  placed  at  some  distance,  the  flash  of 
a  line  of  yellow  light  which  we  shall  presently  learn  is  an 
unfailing  sign  of  the  presence  of  sodium. 

It  was  to  be  expected  that  so  sensitive  a  means  of  investi- 
gation, from  which  no  known  substance  can  escape,  would 
very  soon  lead  to  the  tracking  out  and  discovery  of  new  ele- 
ments which,  till  then,  had  remained  unknown,  either  be- 
cause they  are  scattered  very  sparingly  in  nature,  or  stand 
out  with  so  little  that  is  characteristic,  from  some  other  sub- 
stances, that  the  imperfect  chemical  methods  hitherto  in  use 
have  not  been  able  to  distinguish  them. 

This  expectation  was  brilliantly  realized  even  by  the  first 
steps  taken  in  this  direction.  The  two  Heidelberg  pro- 
fessors, Bunsen  and  Kirchhoff,  to  whom  we  are  indebted  for 
the  discovery  of  spectrum  analysis  and  its  application  to 
practical  science,  very  soon  discovered  with  their  new  instru- 
ment, two  new  metals,  Caesium  and  Rubidium,  to  which 
two  others,  Thallium  and  Indium,  have  been  since  added. 

But  all  the  brilliant  and  astounding  results  which  spectrum 
analysis  has  furnished  in  the  provinces  of  physics  and  chem- 
istry have  been  far  surpassed  by  its  performances  in  that  of 
astronomy.  Newton's  law  of  gravitation  has  given  us  the 
means  of  calculating  the  courses  of  the  heavenly  bodies,  of 
projecting  the  orbits  of  the  earth,  the  planets  and  comets, 
and  of  predicting  their  relative  positions  in  these  orbits, 
together  with  the  accompanying  phenomena  of  the  ebb  and 
flow  of  the  tides,  and  the  eclipses  and  occultations  of  the 

4 


SCHELLEN.]        SPECTRUM  ANALYSIS  EXPLAINED.  59 

heavenly  bodies.  But  this  same  gravitation  chains  man  to 
the  earth,  and  forbids  him  to  leave  it.  It  is  therefore  only 
on  the  wings  of  light,  that  news  reaches  him  of  the  existence 
of  those  numberless  worlds  by  which  he  is  surrounded. 
The  light  alone,  which  proceeds  from  these  stars,  is  the 
winged  messenger  which  can  bring  him  information  of  their 
being  and  nature ;  spectrum  analysis  has  made  this  light 
into  a  ladder  on  which  the  human  mind  can  rise  billions  and 
billions  of  miles,  far  into  immeasurable  space,  in  order  to 
investigate  the  chemical  constitution  of  the  stars,  and  study 
their  physical  conditions. 

Until  within  a  few  years,  the  telescope  was  the  only 
means  by  which  these  investigations  could  be  carried  on, 
and  the  intelligence  derived  from  this  source  concerning  the 
stars  and  nebulas  was  very  scant,  being  confined  to  but  par- 
tial information  of  their  outward  form,  size,  and  color. 

Since  the  year  1859,  spectrum  analysis  has  entered  the 
service  of  astronomy,  and  its  performances  for  the  short 
space  of  eleven  years  are,  in  the  most  widely-differing  ways, 
perfectly  astounding. 

It  is  possible  by  means  of  a  prism  to  decompose  into  its 
component  parts  the  light  of  the  sun,  the  planets,  the  fixed 
stars,  comets  and  nebulas,  and  thus  obtain  their  spectra  iri 
the  same  way  as  that  of  earthly  luminous  substances.  By  a 
careful  comparison  of  the  spectra  of  the  stars  with  the  well- 
known  spectra  of  terrestrial  substances,  it  can  be  determined, 
from  their  complete  agreement  or  disagreement,  with  a  cer- 
tainty almost  amounting  to  mathematical  precision,  whether 
these  substances  do  or  do  not  exist  in  those  remote  heavenly 
bodies. 

The  foregoing  statements  present,  in  general  terms,  the 
essence  and  scope  of  spectrum  analysis.  Its  starting-point 
is  the  spectrum  of  each  individual  substance,  and  in  order 
to  obtain  this  it  is  requisite  that  the  substance  should  not 
only  be  luminous,  but  should  emit  a  sufficient  quantity  of 
light.  Dark  bodies  are  not  available  for  spectrum  analysis  ; 

5 


60  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLEN. 

if  they  are  to  be  submitted  to  its  scrutiny,  they  must  first  be 
brought  into  a  state  of  vivid  luminosity. 

That  the  nature  of  the  analysis  of  light  may  be  more 
easily  understood,  we  will  first  proceed  to  explain 

THE  SOLAR  SPECTRUM. 

If  a  ray  of  sunshine  be  allowed  to  pass  through  a  small, 
round  hole  in  the  window-shutter  of  a  darkened  room,  as  is 
shown  in  Fig.  I.,  there  will  appear  a  round,  white  spot  of 
light  exactly  in  the  direction  of  the  ray,  upon  a  screen  placed 
opposite  the  opening,  as  will  be  seen  indicated  by  the  dotted 
lines  in  the  figure.  A  very  different  appearance  will  be  pre- 
sented if  the  ray  of  light  be  made  to  fall  upon  a  prism. 
The  ray  is  at  once  deflected  from  its  straight  course  upwards, 
that  is  to  say,  towards  the  base  of  the  prism,  and  away  from 
the  sharp  edge  of  the  refracting  surfaces,  which,  as  repre- 
sented in  the  drawing,  are  turned  downwards :  on  its  emer- 
gence from  the  prism  it  no  longer  remains  one  single  ray,  as 
it  entered  the  window-shutter,  but  is  separated  into  very 
many  single-colored  rays,  which,  as  they  continue  to  diverge, 
form  upon  the  screen  an  elongated  band  of  brilliant  colors, 
instead  of  the  former  round,  white  image  of  the  sun.  In 
this  brilliant  band  the  individual  colors  blend  gradually  one 
into  the  other,  beginning  at  thai  end  lying  nearest  the  direc- 
tion of  the  incident  ray  (the  lowest  end  in  the  figure),  with 
the  least  refrangible  color,  a  dark  and  very  beautiful  red ; 
this  passes  imperceptibly  into  orange,  and  orange  again  into 
bright  yellow ;  a  pure  green  succeeds,  which  is  shaded  off 
into  a  brilliant  blue,  and  this  gives  place  to  a  rich,  deep 
indigo ;  a  delicate  purple  leads  finally  to  a  soft  violet,  by 
which  the  range  of  the  visible  rays  is  terminated.  C^A  faint 
picture  of  this  magnificent  solar  image  is  given  in  No.  i  of 
the  Frontispiece  ;  this  is  called  the  spectrum?)  In  the  above- 
mentioned  colors  of  the  solar  spectrum,  the  eye  discerns 
numberless  gradations,  which  pass  imperceptibly  from  one 

6 


SCHELLEN.J        SPECTRUM  ANALYSIS  EXPLAINED.  61 

to  another ;  and  since  language  does  not  suffice  to  give 
separate  names  to  each  of  these,  we  must  content  ourselves 
with  designating  only  the  seven  principal  groups,  which  are 
known  as  the  colors  of  the  spectrum. 

This  experiment  furnishes  conclusive  evidence  that  white 

FIG.  I. 


EXHIBITION   OF   THE   SOLAR   SPECTRUM. 

light  is  not  simple  and  indivisible,  but  composed  of  innu- 
merable colored  rays,  each  of  which  possesses  its  own  pe- 
culiar degree  of  refrangibility,  and  therefore,  on  refraction, 
pursues  a  separate  path.  The  prism  analyzes  white  light ; 
the  result  is  the  separation  of  all  the  colored  rays  of  which 

7 


62  RECREATIONS    IN    POPULAR    SCIENCE.     [SCHELLEN. 

it  is  composed,  and  the  consequent  formation  of  the  colored 
image  called  the  spectruyi., 

The  decomposition  of  sunliglrTby  refraction  is  shown  in 
various  phenomena  known  to  the  ancients  as  well  as  our- 
selves, though  they  were  not  able,  as  we  are,  to  trace  them 
back  to  their  true  cause.  The  rainbow,  with  its  pure  but 
delicate  colors,  the  sparkle  of  the  cut  jewel  in  its  brilliant 
flashes,  the  play  of  color  emitted  by  cut  glass,  and  the  pris- 
matic facets  of  crystal  lustres  as  the  sun  shines  upon  them, 
the  glow  of  the  clouds  and  high  mountain  peaks  in  the 
various  colored  light  of  the  rising  and  setting  sun,  —  all 
these  effects  are  occasioned  by  the  decomposition  of  white 
light  by  its  refraction  on  passing  through  glass  in  a  prismatic 
form,  through  drops  of  liquid,  or  through  vapor. 

The  colors  of  the  solar  spectrum  possess  a  purity  and 
brilliancy  to  be  met  with  nowhere  else ;  they  are  all  per- 
fectly indivisible,  and  cannot  be  further  decomposed,  as  may 
be  easily  proved  on  attempting  to  analyze  a  colored  ray  by 
means  of  a  second  prism.  If  a  small,  round  hole  be  made 
in  the  screen  in  any  portion  of  the  image-of  the  spectrum, — 
the  extreme  red,  for  instance  (Fig.  I.),  —  a  red  ray  passes 
through  it,  and  appears  upon  the  opposite  wall  as  a  round 
spot  of  red  light,  precisely  in  the  same  direction  as  the  red 
rays  left  the  prism  on  the  other  side  of  the  screen.  If  a 
second  prism  be  interposed  in  the  path  of  the  ray  that  has 
passed  through  the  screen,  the  ray  will  suffer  a  second  re- 
fraction, arid  the  image  be  thrown  upon  another  place 
(higher  up  in  the  figure)  on  the  wall ;  this  new  image,  how- 
ever, is  simply  red,  like  the  incident  ray,  and  by  a  careful 
adjustment  of  the  prism  shows  no  elongation,  but  appeals 
perfectly  round. 

This  decomposition  of  sunlight,  or  white  light,  as  it  is 
usually  termed,  is  called  dispersion,  and  is  caused  by  refrac- 
tion, by  which  is  meant  the  deviation  of  a  ray  of  light  from 
a  straight  line  when  it  strikes  any  transparent  substance  (as, 
for  instance,  glass  or  water)  obliquely.  Different  colors  are 

8 


SCHELLEN.]       SPECTRUM  ANALYSIS   EXPLAINED.  63 

said  to  have  different  degrees  of  refrangibility ;  and  by  this 
we  mean,  that  different  colored  rays  of  light,  being  passed 
through  a  triangular  piece  of  glass,  or  prism,  are  turned 
more  or  less  from  the  direction  in  which  they  entered  it. 
Thus  the  colors  of  the  rainbow  have  each  a  different  degree 
of  refrangibility,  from  red,  which  has  the  least,  to  violet, 
which  has  the  greatest ;  and  when  white  light,  which  is 
composed  of  all  these  colors  combined,  is  analyzed,  or  sep- 
arated, these  colors  are  refracted,  or  turned  at  different 
angles,  so  as  to  separate,  though  each  is  beautifully  blended 
with  the  one  next  it.  The  same  is  true  of  light  from  any 
source.  It  is  only  necessary  that  any  substance  should  be 
made  sufficiently  luminous,  to  become  thoroughly  sifted,  or 
analyzed,  by  means  of  the  spectroscope. 

We  will  now  proceed  to  explain  how  all  substances,  such 
as  gases,  metals,  earths,  etc.,  may  be  made  sufficiently 
luminous  to  be  made  available  for  spectrum  analyses.  It  is 
necessary  to  have  sufficient  heat  to  volatilize  or  turn  into 
vapor  any  substance  to  be  analyzed.  For  this  purpose 
many  ingenious  contrivances  have  been  invented,  one  of 
which  was  invented  by  Bunsen,  one  of  the  discoverers  of 
spectrum  analysis.  This  is  called  the  Bunsen  burner,  and 
consists  of  a  gas  burner,  arranged  with  a  chamber  beneath 
it,  where  atmospheric  air  is  mixed  with  the  gas  before  pass- 
ing up  the  tube  to  feed  the  burner.  The  flame  from  this 
burner  is  non-luminous,  but  its  heat  is  intense,  and  may  be 
made  much  greater  if  the  atmospheric  air,  instead  of  being 
left  to  mix  itself  with  the  gas,  be  forced  in  by  means  of  a 
powerful  blowpipe. 

In  the  Bunsen  burner,  the  combustion  of  coal  gas  ensues 
slowly  and  incompletely ;  slowly,  because  the  hydrogen  in 
combination  with  carbon  is  supplied  only  in  small  quantities  ; 
incompletely,  because  the  gases  are  not  mixed  in  due  pro- 
portions, and  the  nitrogen  of  the  air  presents  a  hinderance. 
If,  on  the  contrary,  pure  hydrogen  gas  be  previously  mixed 
with  as  much  pure  oxygen  as  will  insure  its  complete  com- 

9 


64  RECREATIONS  IN  POPULAR   SCIENCE.  [SCHELLBK. 

bustion  (two  volumes  of  hydrogen  with  one  of  oxygen), 
oxyhydrogen  gas  is  obtained,  which,  when  ignited,  explodes 
with  a  fearful  noise,  and  occasions  sometimes  the  destruction 
of  the  strongest  vessels.  The  heat  evolved  by  this  combus- 
tion is  the  greatest  which  can  at  present  be  produced  by 
chemical  means,  and  it  is  sufficient  to  accomplish  the  fusion 
of  substances  which  have  borne  unchanged  the  action  of  the 
hottest  furnaces. 

In  order  to  make  the  oxyhydrogen  flame  a  source  of  in- 
tense light,  a  cylinder  of  well-burnt  lime  is  placed  upon  the 
socket  of  the  lamp,  and  the  flame  directed  against  the  upper 
part.  It  begins  at  once  to  glow,  and  soon  throws  out  a 
dazzling,  incandescent,  or  white-heat  light.  This  is  called 
the  oxyhydrogen  or  Drummond's  lime-light. 

THE  ELECTRIC  SPARK. 

To  attain,  however,  the  greatest  amount  of  heat  and  light 
which  can  at  present  be  produced,  we  must  leave  the  prov- 
ince of  chemistry,  with  its  processes  of  combustion,  and 
turn  to  that  of  electricity,  where  we  are  encountered  by  a 
host  of  phenomena,  accompanied  by  an  intense  degree  of 
light  and  heat. 

Besides  the  well-known  machines  which  excite  electricity 
through  the  friction  of  a  glass  disk,  there  has  been  added  of 
late  a  contrivance  called  an  induction  machine,  which  yields 
a  rich  supply  of  electric  force,  and  gives  a  spark  of  intense 
brilliancy.  In  all  electrical  motors  arranged  for  exhibiting 
light,  sparks  are  formed  between  two  metallic  poles  or  pieces 
of  wire,  which  are  placed  in  contact  with  those  parts  of  the 
machine  which  collect  the  positive  and  negative  electricity. 
By  the  mutual  attraction  of  the  two  electricities,  and  the 
struggle  for  union,  there  ensues  a  tension  of  electricity  at  the 
end  of  the  metal  poles  when  they  are  separated  from  each 
other ;  if  this  be  so  strong  that  the  obstacle  presented  by  the 
stratum  of  air  between  the  metallic  conductors  is  overcome 

10 


SCHELLEN.]      SPECTRUM  ANALYSIS  EXPLAINED.  65 

by  it,  then  the  electricities  are  instantly  united,  and  the 
union  takes  place  in  that  form  of  light  and  heat  which  is 
called  the  electric  spark. 

The  amount  of  heat  thus  generated  depends  upon  the 
degree  of  tension  and  the  quantities  of  electricity  by  the 
union  of  which  it  is  produced ;  but  in  most  cases  it  is  so 
great  that  small  particles  of  the  metal  poles  are  volatilized, 
and  become  luminous.  The  glowing  metallic  vapor  affects 
the  color  of  the  spark,  which  therefore  appears  with  various 
kinds  of  light,  according  to  the  nature  of  the  conductors. 
These  phenomena  afford  us,  in  aid  of  our  researches  with 
spectrum  analysis,  a  very  simple  method  of  volatilizing  and 
raising  to  a  high  degree  of  luminosity  most  of  the  metals, 
and  other  substances  which  are  conductors  of  electricity.  To 
obtain  the  same  result  with  liquids,  it  is  only  necessary  to 
place  one  of  the  metal  poles  in  the  liquid  to  be  examined, 
and  to  bring  the  other  sufficiently  near  the  surface  for  the 
spark  to  pass  from  it  to  the  liquid.  By  the  heat  of  the  spark 
a  small  portion  of  the  liquid  is  volatilized  and  made  luminous. 

THE  VOLTAIC  ARC. 

It  will  be  well  now  to  turn  our  attention  for  a  short  time  to 
that  source  of  electricity  which  is  able  to  evolve  the  highest 
degree  of  heat  with  the  most  intense  light  —  namely,  the  vol- 
taic arc,  or  the  electric  light.  When  the  poles  of  a  power- 
ful voltaic  battery  of  fifty  or  sixty  elements  are  connected,  by 
means  of  two  metal  wires,  with  two  pieces  of  carbon,  «,  b 
(Fig.  II.),  and  these  brought  into  contact,  the  electricity  gen- 
erated by  the  battery  is  discharged  between  them  through 
the  carbon,  which  is  nearly  as  good  a  conductor  as  the 
metal.  If  these  pieces  of  carbon  be  pointed  at  the  ends,  an 
extraordinarily  intense  light  is  emitted  on  the  passage  of  the 
current  at  the  points  of  contact,  and  they  may  be  separated 
one  or  two  tenths  of  an  inch  without  interrupting  the 
discharge.  The  apparatus  is  then  placed  in  the  lantern, 

ii 


66 


RECREATIONS   IN  POPULAR   SCIENCE.     [SCHELLEN 

FIG.  II. 


THE    ELECTRIC    LIGHT. 


which  is  provided  with  requisite  arrangements  to  keep  the 
carbon  points  at  a  proper  distance  from  each  other,  and  also 
with  a  small  mirror  and  lens  to  project  the  light  through  a 
slit  in  the  lantern.  The  complete  arrangement  is  called 


THE  ELECTRIC  LAMP. 

To  exhibit  the  spectrum,  the  room  should  be  darkened, 
and  the  electric  lamp  placed  on  a  table,  with  the  wires  con- 
nected with  a  battery.  (See  Fig.  III.)  The  light  from  the 


12 


SCHELLEN.J       SPECTRUM  ANALYSIS  EXPLAINED.  67 

FIG.  III. 


THE    ELECTRIC    LAMP. 

carbon  points  passes  through  the  narrow,  vertical  slit  in  the 
lamp,  and  by  means  of  the  movable  lens,  D,  a  distinct  image 
of  the  slit  is  thrown  through  the  prisms  E  E  upon  a  screen 
at  a  distance  of  about  twelve  feet,  and  we  behold  a  magnifi- 
cent spectrum,  about  three  feet  long  and  sixteen  inches  wide, 
exhibiting  the  whole  range  of  colors,  as  shown  in  No.  i  of 

13 


68  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHBLLEN. 

the  Frontispiece,  with  this  difference.  In  No.  i,  Frontis- 
piece, the  colors  are  crossed  with  dark  lines,  this  being  a 
feature  of  the  Solar  Spectrum  which  will  be  explained  far- 
ther on.  The  colors  from  the  incandescent  or  white-hot 
carbon  points  succeed  each  other  without  the  slightest  inter- 
ruption. Their  limits  are  not  sharply  defined ;  they  rather 
blend  gradually  one  into  the  other,  and  thus  form  an  un- 
broken or  continuous  spectrum. 

RECOMBINATION  OF  THE  COLORS  OF  THE  SPECTRUM. 

If  white  light  be  actually  composed  of  the  colors  con- 
tained in  the  spectrum,  then  the  recombination  of  the  same 
colors  must  reproduce  white  light.  The  simplest  method  of 
collecting  several  rays  of  light  into  one  point  is  by  a  convex 
lens  or  a  burning-glass.  If  the  sun's  rays  fall  perpendic- 
ularly on  such  a  glass,  the  refraction  they  suffer  in  their 
passage  through  it  causes  them  to  converge  to  one  point  — 
the  focus.  To  accomplish  by  this  means  the  recombination 
of  the  colored  rays  of  the  spectrum  of  the  electric  light,  a 
cylindrical  lens  must  be  interposed  between  the  prism  and 
the  screen  on  which  the  spectrum  of  the  small  line  of  light 
issuing  from  the  slit  is  extended  to  a  length  of  some  six  feet : 
this  lens  is  a  convex  lens  of  peculiar  form,  which  possesses 
the  property  of  recombining  in  a  point  all  the  rays  issuing 
from  each  point  of  the  line  of  light  passing  through  the  slit 
after  dispersion  by  the  prism,  and  therefore  of  representing 
the  whole  of  the  rays  of  that  short  line  of  light  again  as  a 
small  line.  When,  therefore,  this  lens  is  placed  at  a  proper 
distance  behind  the  prism,  the  colors  of  the  spectrum  disap- 
pear from  the  screen,  and  are  replaced  by  a  short  line  of 
light,  in  which  all  the  colored  rays  issuing  from  the  prism 
have  been  recombined,  and  the  white  light  reproduced  out 
of  which  they  originated. 

"4 


SCHELLEN.]       SPECTRUM  ANALYSIS   EXPLAINED.  69 


THE  CONTINUOUS  SPECTRA  OF  SOLID  AND  LIQUID  BODIES. 

When  the  carbon  points  used  for  the  production  of  the 
electric  light  are  carefully  prepared,  and  completely  free 
from  all  extraneous  substances,  the  light  is  purely  white, 
being  emitted  exclusively  by  solid  particles  of  carbon  in  a 
state  of  incandescence.  The  spectrum  of  this  light  is,  there- 
fore, continuous,  like  that  of  incandescent  lime;  it  is  un- 
broken by  gaps  in  the  colors,  or  by  sudden  transitions  from 
one  color  to  another,  and  is  uninterrupted  by  either  dark 
or  bright  bands. 

All  other  incandescent  bodies,  whether  solid  or  liquid, 
give  a  similar  spectrum,  the  colors  being  distributed  in  the 
order  represented  in  the  Frontispiece,  No.  I.  If,  instead  of 
the  lime-light,  the  magnesium  light,  the  light  of  an  incandes- 
cent platinum  wire,  or  the  flame  of  coal  gas  in  which  light 
is  produced  by  incandescent  particles  of  carbon,  be  analyzed 
by  the  prism,  continuous  spectra  are  always  obtained,  but 
with  this  difference,  that  the  various  groups  of  color  are  not 
always  distributed  in  exactly  the  same  proportion  in  each 
individual  spectrum  ;  and  therefore,  according  to  the  kind 
of  light  employed,  sometimes  red,  sometimes  yellow,  and 
sometimes  violet  predominates.  Therefore,  where  there  is 
a  continuous  spectrum  without  gaps,  and  containing  every 
shade  of  color,  the  light  is  derived  from  an  incandescent 
solid  or  liquid  body. 

THE  SPECTRA  OF  VAPORS  AND  GASES. 

Very  different  spectra  are  obtained  when  the  source  of 
light  is  not  an  incandescent  solid  or  liquid  body,  but  a  vapor 
or  a  gas  in  a  glowing  state.  Instead  of  a  continuous  succes- 
sion of  colors,  the  spectrum  then  exhibits  a  series  of  distinct 
bright-colored  bands,  separated  one  from  another  by  dark 
spaces. 

15 


70  RECREATIONS    IN  POPULAR    SCIENCE.    [SCHELLEN. 

The  characteristic  feature  of  spectra  obtained  from  lumi- 
nous vapors  or  gases  is  the  want  of  continuity  in  the  succes- 
sion of  the  colors.  Such  a  spectrum  is  composed  of  distinct 
colored  bands,  irregularly  arranged,  with  dark  spaces  be- 
tween them,  and  is  therefore  called  a  discontinuous  spec- 
trum, a  spectrum  of  bright  lines,  or  a  gas  spectrum. 

The  spectra  of  the  vapors  of  potassium,  sodium,  caesium, 
and  rubidium  are  represented  in  Nos.  2,  3,  4,  and  5  of  the 
Frontispiece. 

The  colored  plate  at  the  beginning  shows  certain  spectra, 
observed  by  means  of  the  spectroscope.  It  contains  five 
specimens  of  discontinuous  or  gas  spectrums.  Fig.  I.  rep- 
resents the  solar  spectrum,  which  is,  as  we  showed  before, 
simple  white  light,  or  sunlight  analyzed  into  its  constituent 
parts.  Fig.  II.  shows  the  spectrum  of  potassium.  It  is  con- 
tinuous, with  the  exception  of  the  line  in  the  extreme  red, 
and  another  in  the  extreme  violet,  and  without  these  two 
lines,  would  show  the  spectrum  of  an  incandescent  solid  or 
liquid  body,  or,  in  other  words,  a  body  raised  to  the  point  of 
white  heat.  Fig.  III.  shows  the  spectrum  of  sodium  or  salt. 
This  spectrum  contains  neither  red,  orange,  green,  blue,  nor 
violet.  It-is  marked  by  a  very  brilliant  yellow  ray.  Of  all 
metals,  sodium  is  that  which  possesses  the  greatest  spectral 
sensibility.  In  fact,  it  has  been  ascertained  that  one  two- 
hundred  millionth  of  .a  grain  of  salt  is  enough  to  cause  the 
appearance. of  the  yellow  line  of  sodium.  A  very  little  dust 
scattered  in  the  room  is  enough  to  produce  it :  a  circumstance 
which  shows  how  abundantly  sodium  is  scattered  throughout 
nature.  Kirchhoff  has  also  ascertained  that  it  exists  in  the 
sun  and  the  fixed  stars. 

Figures  IV.  and  V.  show  the  spectra  of  ccesium  and  rubid- 
ium, metals  discovered  by  Bunsen  and  Kirchhoff,  by  means 
of  Spectrum  Analysis.  The  former  is  distinguished  by  two 
blue  lines, .the  latter  by  two  brilliant  red  lines,  and  two  less 
intense  violet  lines. 

16 


SCHELLEN.]       SPECTRUM  ANALYSIS  EXPLAINED.  71 

Two  other  metals,  thallium  and  indium,  have  lately  been 
discovered  by  other  scientists  by  the  same  means. 

Terrestrial  substances  must  be  volatilized,  or  made  into 
vapor,  to  be  examined.  The  spectra  of  incandescent  solid 
and  liquid  bodies  are  continuous,  and  resemble  each  other 
so  closely,  that  only  in  a  very  few  instances  can  they  be  dis- 
tinguished ;  spectra  of  this  kind  are,  therefore,  not  suitable 
for  the  recognition  of  a  substance,  though  they  authorize  the 
conclusion,  as  a  rule,  that  the  substance  is  either  in  a  solid 
or  liquid  state.  Only  the  discontinuous  spectra,  consisting 
of  colored  lines  which  are  obtained  from  a  gas  or  vapor,  are 
sufficiently  characteristic  to  enable  the  observer  to  pronounce 
with  certainty,  by  the  number,  position,  and  relative  bright- 
ness of  these  lines,  the  chemical  constitution  of  the  vapors 
by  which  the  light  has  been  emitted.  It  follows  from  this 
circumstance  that  spectrum  analysis  deals  pre-eminently 
with  the  investigation  of  gas  spectra,  and  that  for  the  exam- 
ination of  a  substance  which  does  not  exist  in  nature  in  the 
form  of  gas  or  vapor,  the  first  step  must  be  to  place  it  in 
this  condition.  We  explained  how  this  is  done  by  the  use 
of  the  Electric  Lamp  and  the  Drummond  Lime-light. 

LIGHT. 

Although  the  theory  of  light  is  now  so  completely  under- 
stood that  we  are  able  to  explain  the  most  complicated  opti- 
cal phenomena,  yet  an  elementary  reply  to  the  question, 
What  is  the  nature  of  light?  still  presents  some  difficulty. 
We  perceive  the  operation  of  this  power  of  nature  in  all 
directions,  and  in  the  most  manifold  ways;  the  sun,  as  it 
stands  in  full  splendor  in  the  heavens,  pours  forth  but  a 
single  tone  of  color  over  the  earth,  and  yet  the  individual 
objects  in  the  landscape  appear  in  the  most  varied  and  glo- 
rious tints.  What  then  are  these  colors?  How  are  they 
developed  out  of  the  white  light  which  the  sun  and  other 
luminous  bodies  emit? 

17 


72  RECREATIONS  IN  POPULAR  SCIENCE.     [SCHELLRK 

We  need  not  seek  to  avoid  answering  this  question,  if  we 
can  succeed  in  giving  a  clear  insight  into  the  phenomena  of 
spectrum  analysis ;  for  we  have  already  intimated  that  the 
world  of  color  is  the  peculiar  province  of  this  new  method 
of  investigation. 

The  approaches  to  science  are  frequently  obstructed  by 
strange  propositions^  discouraging  and  apparently  contradic- 
tory, which  seem  to  the  uninitiated  like  those  ghosts  that 
haunted  the  way  by  which  Dante  and  his  heavenly  guide 
descended  to  the  realms  of  the  departed ;  with  a  little  cour- 
age, however,  we  may  easily  traverse  this  dreaded  path, 
seize  hold  of  the  harmless  apparitions,  and  make  friends 
first  with  one  and  then  with  another  as  we  approach  them. 

We  will  therefore  boldly  grasp  the  proposed  inquiry ;  if 
the  answer  to  it  cannot  be  exhaustive,  it  will  at  least  contain 
material  enough  to  incite  to  further  reflection,  and  perhaps 
also  afford  the  necessary  basis  for  a  more  easy  comprehen- 
sion of  the  elaborate  theories  which  are  enunciated  in  phys- 
ical treatises. 

According  to  the  theory  generally  received  at  present,  the 
whole  universe  is  an  immeasurable  sea  of  highly  attenuated 
matter,  imperceptible  to  the  senses,  in  which  the  heavenly 
bodies  move  with  scarcely  any  impediment.  This  fluid, 
which  is  called  ether,  fills  the  whole  of  space  —  fills  the 
intervals  between  the  heavenly  bodies,  as  well  as  the  pores 
or  interstices  between  the  atoms  of  a  substance.  The  small- 
est particles  of  this  subtle  matter  are  in  constant  vibratory 
motion ;  when  this  motion  is  communicated  to  the  retina  of 
the  eye,  it  produces,  if  the  impression  upon  the  nerves  be 
sufficiently  strong,  a  sensation  which  we  call  light. 

Every  substance,  therefore,  which  sets  the  ether  in  power- 
ful vibration,  is  luminous ;  strong  vibrations  are  perceived 
as  intense  light,  and  weak  vibrations  as  faint  light,  but  both 
of  them  proceed  from  the  luminous  object  at  the  extraordi- 
nary speed  of  186,000  miles  in  a  second,  and  they  necessa- 

iS 


SCHELLEN.]       SPECTRUM  ANALYSIS  EXPLAINED.  73 

rily  diminish  in  strength  in  proportion  as  they  spread  them- 
selves over  a  greater  space. 

Light  is  not  therefore  a  separate  substance,  but  only  the 
vibration  of  a  substance,  which,  according  to  its  various 
forms  of  motion,  generates  light,  heat,  or  electricity. 

ANALOGY  BETWEEN  LIGHT  AND  SOUND. 

This  representation  of  the  nature  of  light  ceases  to  be  sur- 
prising when  we  come  to  compare  the  vibrations  of  ether 
with  those  of  atmospheric  air,  and  draw  a  parallel  between 
light  and  sound  —  between  the  eye  and  the  ear. 

A  string  set  in  vibration  causes  a  compression  and  rarefac- 
tion of  the  surrounding  air ;  in  front  of  it  the  air  is  pushed 
together  and  condensed ;  behind  it  the  vacuum  it  creates  is 
filled  up  by  the  surrounding  air,  which  thus  becomes  rarefied 
for  the  moment.  This  periodic  movement  of  the  air  is 
transmitted  to  our  ears  at  the  rate  of  about  1,100  feet  in  a 
second  ;  it  strikes  against  the  tympanum,  and  occasions,  by* 
its  further  impulse  on  the  auditory  nerves  and  brain,  the  sen- 
sation we  call  sound.  Air  in  motion,  by  its  influence  on  the 
organs  of  hearing,  is  the  cause  of  sound  ;  ether  in  motion, 
by  its  influence  on  the  organs  of  sight,  is  the  cause  of  light. 
Without  air,  or  some  other  medium  whereby  the  vibration 
of  bodies  can  be  propagated  to  our  ears,  no  sound  is  pos- 
sible. As  a  sonorous  body  throws  oft'  no  actual  substance 
of  sound,  but  only  occasions  a  vibration  of  the  air,  so  «  lu- 
minous body  sends  out  no  substance  of  light,  but  only  gives 
an  impulse  to  the  ether,  and  sets  it  in  vibration. 

A  musical  sound,  in  contradistinction  to  mere  noise,  is 
produced  vonly  when  the  impulses  of  the  air  reach  the  ear  at 
regular  intervals ;  if  the  intervals  between  the  impulses  are 
not  sufficiently  regular,  the  ear  is  only  conscious  of  a  hissing, 
a  rushing,  or  a  humming  noise ;  a  musical  sound  requires 
perfect  regularity  in  the  succession  of  impulses. 

The  pitch  of  a  musical  note  depends  on  the  number  of 

'9 


/4  RECREATIONS   IN   POPULAR   SCIENCE.  [SCHELLEX. 

impulses  in  a  given  time  —  as,  for  instance,  in  a  second ;  the 
greater  the  number  of  vibrations  in  a  second,  the  higher  will 
be  the  note  produced.  When  the  single  impulses  are  fewer 
than  sixteen  or  more  than  forty  thousand  in  a  second,  the 
ear  is  no  longer  sensible  of  a  musical  sound :  in  the  first 
case,  it  either  perceives  only  an  undefined,  deep  hum,  or 
else  it  distinguishes  the  individual  strokes  upon  the  tym- 
panum, and  becomes  sensible  of  them  as  distinct  blows ;  in 
the  latter  case,  there  is  an  impression  of  a  sharp,  but  equally 
indefinite  shrill  or  hissing  noise.  The  limits  of  susceptibility 
of  the  ear  for  musical  sounds  lie  between  sixteen  and  forty 
thousand  impulses  per  second.  The  number  of  vibrations 
in  a  second  given  by  a  normal  tuning-fork  was  determined 
in  the  year  1859  *°  ke  435  *n  a  temperature  of  15°  C. 

(59°  F.)» 

The  truth  of  the  foregoing  statements  may  be  easily  proved 
in  the  following  manner.  A  disk  of  zinc  is  fastened  to  an 
axis  which  can  be  set  in  rapid  rotation  by  means  of  a  cord 
.working  over  a  large  wheel.  The  disk  is  perforated  with 
eight  series  of  holes,  placed  along  eight  concentric  circles ; 
the  holes  are  of  the  same  size  in  each  circle,  and  at  equal 
distances  from  each  other,  so  that  their  number  increases  in 
each  ring  from  the  centre  to  the  edge. 

When  the  disk,  by  means  of  the  large  wheel,  is  set  in  uni- 
form motion  at  the  rate  of  one  revolution  in  a  second,  and 
one  circle  of  the  holes  is  blown  upon  with  considerable  force 
through  a  glass  or  metal  tube,  a  note  is  heard :  by  blowing 
upon  the  next  series  higher,  the  note  is  of  a  higher  pitch  ;  a 
lower  set  of  holes  gives,  on  the  contrary,  a  deeper  note ;  so 
that  if  all  the  rings  were  blown  upon  in  succession,  from  the 

*  [The  number  of  vibrations  of  a  C  tuning-fork  is  512.  The 
deepest  tone  of  orchestral  instruments  is  the  E  of  the  double  bass, 
•with  41^  vibrations.  Some  organs  go  as  low  as  C'with  33  vibra- 
tions, and  some  pianos  may  reach  A  with  27^  vibrations.  In  height 
the  piano-forte  reaches  to  aiv  with  3-520.  The  highest  note  of  or- 
chestra is  probably  dv  of  the  piccolo  flute  wrth  4752  vibrations.] 

20 


SCHKLLEN.]      SPECTRUM  ANALYSIS   EXPLAINED.  75 

lowest  upwards,  the  distinct  notes  of  the  complete  octave 
would  be  heard. 

What  is  it  that  here  produces  the  sound?  The  mere  rev- 
olution of  the  disk  makes  no  noise ;  the  motion  of  the  air 
by  the  blowing  through  the  tube  first  elicits  the  notes. 
When,  by  the  rotation  of  the  disk,  the  current  of  air  strikes 
against  an  opening,  it  presses  through  it,  pushing  the  air 
before  it  and  condensing  it ;  this  impulse  reaches  the  ear  at 
once,  and  strikes  upon  the  tympanum :  the  current  of  air 
immediately  afterwards  comes  against  the  solid  part  between 
the  holes,  by  which  it  is  interrupted.  If  the  circle  blown 
upon  contain  twenty-four  openings,  the  ear  would  receive 
twenty-four  impulses  at  every  revolution  of  the  disk ;  and 
if  the  disk  made  twenty  revolutions  in  a  second,  the  ear 
would  receive  20  X  24  =480  impulses  in  the  same  interval. 
The  outside  circle  has  twice  as  many  openings  as  the  inner- 
most one  ;  it  therefore  furnishes  with  the  same  speed  of  rota- 
tion 20  X  48  =  960  impulses  in  a  second. 

The  ear  cannot  distinguish  individual  impulses  when  they 
exceed  sixteen  in  a  second ;  the  impressions  they  then  pro- 
duce become  blended  together,  the  one  following  the  other 
so  instantly  that  the  sensation  in  the  ear  is  that  of  one  con- 
tinuous impulse  or  sound. 

The  pitch  of  a  note  is  thus  seen  to  depend  entirely  upon 
the  number  of  successive  impulses  following  each  other  at 
the  same  uniform  rate,  its  strength  upon  the  force  of  the 
impulse.  With  a  stronger  blast,  the  pitch  of  the  note  re- 
mains unchanged,  but  the  'tone  becomes  more  piercing, 
while  if  a  ring  containing  a  greater  number  of  holes  be 
blown  upon,  the  pitch  rises  till  in  the  last  circle,  with  double 
the  number  of  openings,  the  octave  of  the  same  note  is 
heard  that  was  given  by  the  innermost  circle. 

It  is  true  that  the  cause  of  sound  is  not  the  same  in  all 
musical  instruments  ;  sometimes  it  is  the  vibration  of  strings, 
or  elastic  prongs,  sometimes  stretched  membranes,  or,  again, 
columns  of  air  confined  in  tubes  which  create  at  regular 

21 


76  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLBK. 

periods  a  condensation  and  rarefaction  of  the  air ;  but  in 
every  case  a  note  can  only  be  produced  by  similar  impulses 
recurring  at  regular  intervals,  conveyed  by  the  air  to  the 
organs  of  hearing. 

Savart  exhibited  the  cause  of  sound  in  another  way  which 
is  not  less  instructive  than  the  one  just  described.  Instead 
of  the  perforated  disk,  he  made  use  of  a  wheel  provided 
with  six  hundred  teeth,  which  could  be  set  in  very  rapid 
rotation  in  the  same  manner  as  the  disk,  and  as  the  wheel 
revolved,  the  teeth  were  allowed  to  press  against  the  edge 
of  a  card.  .  To  make  this  experiment,  it  is  only  necessary 
to  substitute  a  toothed  or  cog  wheel  for  the  perforated  disk, 
and  while  the  wheel  is  in  rapid  revolution,  to  hold  a  thin 
card,  or  a  piece  of  pasteboard  against  its  toothed  edge.  The 
card  is  bent  a  little  by  each  tooth  as  it  goes  by,  and  springs 
back  to  its  first  position  as  soon  as  it  is  released  by  the  pass- 
ing of  the  tooth :  the  motion  of  the  card  is  communicated 
to  the  surrounding  air,  and  reaches  the  ear  in  consequence 
of  the  regular  revolution  of  the  wheel,  in  the  form  of  waves 
of  air,  or  of  condensations  and  rarefactions  of  the  air  fol- 
lowing each  other  at  regular  intervals. 

When  the  wheel  is  turned  slowly,  there  is  heard  only  a 
succession  of  taps,  or  isolated  impulses  of  the  card,  distinctly 
separable  one  from  another,  which  do  not  as  yet  unite  to 
form  a  musical  sound.  In  proportion,  however,  as  the  ra- 
pidity of  the  rotation  is  increased,  the  number  of  impulses 
increases  also,  and  they  unite  in  the  ear  to  produce  musical 
notes  rising  continually  in  pitch.  A  small  recording  appa- 
ratus, fixed  to  the  axle  of  the  toothed  wheel,  gives  the  num- 
ber of  revolutions  in  a  second ;  if  this  number  be  multiplied 
by  six  hundred,  the  number  of  teeth  on  the  wheel,  the  result 
gives  the  number  of  condensations  of  air  striking  the  ear  in 
a  second.  It  is  easy  by  this  means  to  determine  the  number 
of  vibrations  the  ear  receives  in  a  second  from  a  note  of  any 
given  pitch,  and  thus  to  verify  the  results  obtained  by  the 
perforated  disk. 

22 


SCHELLEN.J       SPECTRUM  ANALYSIS  EXPLAINED.  77 

It  will  now  be  easier  to  understand  the  motion  of  ether, 
and  its  mode  of  operation  on  the  organs  of  sight.  Ether,  as 
well  as  air,  can  be  set  in  regular  vibrations,  and  even  in  such 
a  manner  that  the  phases  of  condensation  and  rarefaction 
are  repeated  at  regular  periods  of  time.  The  difference 
between  the  vibrations  of  the  air  and  the  ether  is  occasioned 
by  the  remarkable  delicacy  and  elasticity  of  the  latter,  which 
not  only  permits  a  greater  rapidity  in  the  propagation  of 
motion  than  is  possible  with  the  coarse  and  heavy  particles 
of  air,  but  also  allows  the  number  of  vibrations  per  second 
to  be  immensely  greater,  so  that  their  number  has  to  be 
reckoned  by  billions. 

ANALOGY  BETWEEN  MUSICAL  SOUNDS  AND  COLORS. 

Colors  are  to  the  eye  what  musical  tones  are  to  the  ear. 
A  certain  number  of  ether  impulses  in  a  second  against  the 
retina  of  the  eye  are  necessary  to  produce  the  sensation  of 
light :  if  the  number  of  these  waves  pass  above  or  below  a 
certain  limit,  the  eye  is  no  longer  sensible  of  them  as  light. 

The  first  sensation  of  these  vibrations  on  the  part  of  the 
eye  commences  at  about  four  hundred  and  fifty  billion  im- 
pulses in  a  second,  and  the  eye  ceases  to  perceive  them  when 
they  have  reached  double  this  number,  or  about  eight  hun- 
dred billion :  in  the  first  case,  the  impression  produced  is 
that  of  dark  red  ;  in  the  latter,  of  deep  violet. 

The  greater  the  number  of  vibrations  in  any  given  time, 
the  more  rapidly  must  the  single  impulses  succeed  each 
other ;  it  may  be  concluded,  therefore,  that  the  different 
colors  are  only  produced  by  the  different  degrees  of  rapidity 
with  which  the  ether  vibrations  recur,  just  as  the  various 
notes  in  music  depend  upon  the  rapidity  of  the  succession 
of  vibrations  of  air.  The  vibrations  which  recur  most 
slowly,  —  amounting,  however,  to  at  least  four  hundred  and 
fifty  billion  in  a  second,  —  give  the  sensation  of  red ;  those 
recurring  more  rapidly  produce  that  of  yellow ;  and  if  the 
'  23 


78  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLKN 

rapidity  with  which  the  impulses  succeed  each  other  con- 
tinue to  increase,  the  sensation  becomes  in  succession  green, 
blue,  and  violet,  with  which  last  color  the  human  eye  be- 
comes insensible  to  the  ether  motion,  which,  however,  is 
still  very  far  from  having  attained  its  limit  of  rapidity.  . 

The  gradation  of  the  colors  from  red  through  yellow, 
green,  and  blue,  to  violet,  is  to  the  eye  what  the  gamut  is  to 
the  ear  ;  and  it  is  therefore  not  without  reason  that  we  speak 
of  the  tone  and  harmony  of  color.  To  the  physicist  the 
words  color  and  tone  are  only  different  modes  of  expression 
for  similar  and  closely  allied  phenomena  ;  they  express  the 
perception  of  regular  movements  recurring  in  equal  periods 
of  time,  —  in  ether,  producing  colors  ;  in  air,  musical  sounds  ; 
in  the  former  instance,  by  means  of  the  organs  of  sight ;  in 
the  latter,  by  the  organs  of  hearing,  —  movements  of  extreme 
rapidity  in  ether,  of  more  moderate  speed  in  air. 

But  it  will  be  asked  what  becomes  of  those  vibrations 
which  are  above  and  below  the  limits  of  the  eye's  sensibility 
to  light  and  color?  Do  they  wander  about  purposeless  and 
unnoticed?  By  no  means:  forces  are  proved  to  exist  in  the 
rays  of  the  sun,  and  other  intensely  luminous  bodies,  which 
cannot  be  perceived  by  the  eye.  Those  slower  vibrations 
which,  though  they  are  reckoned  by  billions  in  a  second,  do 
not  yet  amount  to  four  hundred  and  fifty  billion,  are  made 
apparent  to  us  in  the  sensation  of  heat,  which  is  also  the 
result  of  oscillatory  movement  —  radiant  heat  being,  like 
light,  propagated  without  the  aid  of  foreign  bodies.  Those 
vibrations,  on  the  other  hand,  which  have  a  velocity  greater 
than  that  by  which  deep  violet  is  produced  —  at  which  color 
the  eye's  susceptibility  to  light  ceases  —  reveal  themselves  by 
their  powerful  chemical  action  ;  they  succeed  each  other  too 
rapidly  for  the  visual  nerves  to  be  any  longer  conscious  of 
the  impulses,  but  they  have  the  power  of  working  chemical 
changes,  and  the  decomposition  of  various  substances  can 
be  undoubtedly  traced  to  the  agency  of  these  invisible  rays. 
An  English  physicist  has  succeeded  in  moderating  the  ex- 

24 


SCHKLLHN.]      SPECTRUM  ANALYSIS   EXPLAINED.  79 

cessive  velocity  of  these  vibrations  by  means  of  certain  sub- 
stances, and  in  this  way  has  brought  some  of  the  invisible 
chemical  rays  within  reach  of  the  eye's  susceptibility.* 

Dove  describes,  in  his  own  ingenious  manner,  the  course 
of  the  vibrations  as  they  produce  successively  sound,  heat, 
and  light,  as  follows :  — 

"  In  the  middle  of  a  large,  darkened  room  let  us  suppose 
a  rod,  set  in  vibration  and  connected  with  a  contrivance  for 
continually  augmenting  the  speed  of  its  vibrations.  I  enter 
the  room  at  the  moment  when  the  rod  is  vibrating  four  times 
in  a  second.  Neither  eye  nor  ear  tell  me  of  the  presence  of 
the  rod,  only  the  hand,  which  feels  the  strokes  when  brought 
within  their  reach.  The  vibrations  become  more  rapid,  till 
when  they  reach  the  number  of  thirty-two  in  a  second,!  a 
deep  hum  strikes  my  ear.  The  tone  rises  continually  in 
pitch,  and  passes  through  all  the  intervening  grades  up  to 
the  highest,  the  shrillest  note ;  then  all  sinks  again  into  the 
former  grave-like  silence.  While  full  of  astonishment  at 
what  I  have  heard,  I  feel  suddenly  (by  the  increased  velocity 
of  the  vibrating  rod)  an  agreeable  warmth,  as  from  a  fire, 
diffusing  itself  from  the  spot  whence  the  sound  had  pro- 

*  [Fluorescent  substances  possess  this  property.  The  peculiar 
blue  light  diffused  from  a  perfectly  colorless  solution  of  sulphate  of 
quinine  was  observed  by  Sir  John  Herschel,  and  the  colored  light 
diffused  from  various  vegetable  solutions  and  essential  oils  was  sub- 
sequently examined  by  Sir  David  Brewster.  To  Professor  Stokes, 
however,  is  due  the  true  explanation  of  these  phenomena;  he 
shoved  that  the  blue  light  of  the  solution  of  quinine  consists  of 
vibrt  tions  brought  within  the  limits  of  the  power  of  the  eye  which 
were  originally  too  rapid  to  be  visible.  If  a  fresh  infusion  of  the 
bark  of  the  horse-chestnut  be  placed  beyond  the  limits  of  the  visible 
spectrum  of  sunlight  admitted  through  a  slit  into  a  dark  room,  it 
becomes  beautifully  luminous,  in  consequence  of  the  power  which 
it  possesses  to  lower  the  invisible  ultra-violet  vibrations  into  light 
which  can  affect  the  eye.] 

t  That  is  to  say,  the  tympanum  is  pressed  in  sixteen  times,  and 
sixteen  times  withdrawn;  therefore  sixteen  blows  are  received  upon 
the  ear. 

25 


So  RECREATIONS   IN   POPULAR   SCIENCE.    [SCHELLBN. 

ceeded.  Still  all  is  dark.  The  vibrations  increase  in  rapid- 
ity, and  a  faint  red  light  begins  to  glimmer ;  it  gradually 
brightens  till  the  rod  assumes  a  vivid  red  glow,  then  it  turns 
to  yellow,  and  changes  through  the  whole  range  of  colors 
up  to  violet,  when  all  again  is  swallowed  up  in  night. 
Thus  nature  speaks  to  the  different  senses  in  succession ;  at 
first,  a  gentle  word,  audible  only  in  immediate  proximity, 
then  a  louder  call  from  an  ever-increasing  distance,  till 
finally  her  voice  is  borne  on  the  wings  of  light  from  regions 
of  immeasurable  space." 

THE  COLORS  OF  NATURAL  OBJECTS. 

Besides  the  colors  of  the  spectrum,  which  are  the  simple 
elements  composing  white  light,  there  is  another  class  of 
colors  apparent  in  every  substance,  wThich  are  therefore 
known  as  the  colors  of  natural  objects.  When  we  see  that 
a  picture  is  formed  by  covering  the  canvas  with  various  pig- 
ments, and  that  leaves  and  flowers  are  bright  with  the  most 
beautiful  tints,  while  white  cloth  becomes  red,  green,  or  blue, 
according  to  the  color  of  the  liquid  into  which  it  is  dipped, 
we  are  easily  led  to  believe  that  every  substance  carries  in 
itself  its  own  color,  which  is  peculiar  to  it  alone,  and  is 
inherent  in  the  substance.  At  most,  we  might  admit  that 
light  was  requisite  to  render  the  color  visible. 

And  yet  this  is  not  so.  Were  colors  really  something  in- 
herent in  the  object,  every  colored  substance  would  mani- 
festly appear  always  of  the  same  color,  by  whatever  light  it 
was  illuminated.  But  this,  as  every  one  knows,  is  not  the 
case.  The  beautiful  violet  dress,  which  in  daylight  appears 
of  the  purest  color,  seems  dull  and  gloomy  by  gaslight ; 
materials  which  in  daylight  are  a  bright  blue,  are  tinged 
with  green  in  candle  or  lamp  light.  And  what  if  the  land- 
scape, or  a  colored  object,  be  viewed  through  a  tinted  glass? 
All  colors  then  seem  changed,  without  the  objects  in  them- 
selyes  being  altered  ;  if  the  color  of  the  glass  be  intense,  the 

26 


SCHELLEN.]       SPECTRUM  ANALYSIS   EXPLAINED.  Si 

various  colors  of  the  objects  immediately  disappear,  and 
everything  seems  shaded  in  the  color  of  the  glass.  The 
same  thing  happens  if  some  common  salt  be  rubbed  into  the 
wick  of  a  spirit  lamp,  and  surrounding  objects  viewed  by 
the  yellow  light  of  such  a  flame ;  the  colors  disappear,  or 
lose  much  of  their  brilliancy,  and  everything  seems  either 
in  mere  light  and  shade,  or  else  of  a  dull  gray. 

These  facts  clearly  prove  that  colors  are  not  inherent  in 
objects,  that  they  have  no  independent  existence,  but  that 
they  are  called  forth  by  some  extraneous  cause. 

On  the  other  hand,  these  considerations  show  that  there 
must  be  something  in  the  objects  themselves  to  help  in  the 
formation  of  color ;  for  they  in  no  way  assume  the  color  of 
the  light  illuminating  them,  but  appear,  as  a  rule,  of  quite 
a  different  hue. 

The  natural  color  of  an  object  is  that  in  which  it  appears 
when  illuminated  by  the  pure  white  light  of  the  sun,  or  by 
daylight ;  it  is  called  red  or  blue  when  it  so  appears  by  day- 
light. Now  if  an  object  be  illuminated  by  white  light,  and 
yet  appear  of  another  color,  the  cause  of  the  change  must 
b'e  looked  for  in  the  influence  which  the  surface  of  the  body 
exercises  on  the  ether  waves  constituting  white  light.  The 
effects  of  this  influence  are  very  different,  according  to  the 
nature  of  the  coloring  matter  with  which  the  object  is  pro- 
vided ;  but  they  may  mostly  be  reduced  to  one  of  two  cases 
—  either  that  a  portion  of  the  ether  motion  is  entirely 
stopped,  or  so  considerably  diminished  in  its  passage  over 
the  ponderable  atoms  of  the  substance,  as  that  heat,  instead 
of  light,  is  evolved,  —  or  else  that  the  ether  waves  are  irreg- 
ularly reflected  from  the  surface  of  the  object,  as  sometimes 
occurs  with  the  waves  of  sound.  In  the  first  case,  the  rays 
of  light  are  said  to  be  absorbed;  in  the  latter,  scattered. 

When  the  surface  of  a  body  has  the  property  of  absorbing 
all  the  colors  of  the  solar  spectrum  with  the  exception  of 
one,  —  the  red,  for  example,  —  that  body  appears  red  to  us 
by  daylight,  because  this  color  alone  is  reflected  to  the  eye. 

27 


82  RECREATIONS   IN   POPULAR   SCIENCE.  [SCHELLEJI. 

When,  on  the  contrary,  it  has  the  power  of  absorbing  some 
of  the  rays,  —  the  red  and  orange,  for  instance,  —  and  of 
reflecting  the  others,  namely,  the  yellow,  green,  and  blue, 
the  color  of  the  object  will  then  be  that  produced  by  the 
mixture  of  the  unabsorbed  —  the  reflected  —  colors.  Now 
as  white  light  contains  the  whole  range  of  colors  visible  in 
the  spectrum,  it  can  easily  be  understood  why  so  many  dif- 
ferent colored  objects  should  be  seen  in  nature  with  such  an 
infinite  variety  of  tints.* 

When  all  the  colors  of  white  light  are  reflected  from  an 
object  in  the  same  proportions  as  they  occur  in  the  solar 
spectrum,  the  object  appears  white  by  daylight,  and  brilliant 
in  proportion  to  the  quantity  of  light  it  reflects.  In  propor- 
tion, however,  as  it  reflects  fewer  rays  of  all  kinds,  the 
white  loses  in  intensity ;  the  object  appears  first  gray,  then 
dark,  and  at  last  black,  when  all  the  rays  falling  upon  it  are 
absorbed,  and  none  reflected. 

Those  objects  are  therefore  black  the  surfaces  of  which 
are  so  constituted  as  to  absorb  all  the  colored  rays  of  white 
light ;  those  are  white  which  reflect  all  the  rays  which  fall 
upon  the  surface ;  and  those  are  colored  which  reflect  some 
of  the  rays  and  absorb  others. 

A  white  object  may  therefore  appear  of  all  colors :  if  red 
light  falls  upon  it,  it  reflects  it  to  the  eye,  and  appears  red  ; 
in  blue  light,  it  appears  blue ;  in  green  light,  green,  etc. ; 
whereas  a  black  object  always  appears  black,  whatever  may 
be  the  color  of  the  light  by  which  it  is  illuminated. 

We  may  here  further  remark  that  a  colored  substance 
assumes  a  different  tint  when  illuminated  by  colored  light, 
and  then  appears  of  another  than  its  natural,  that  is  to  say, 
daylight  color.  Vermilion,  for  example,  when  placed  in 
red  light,  becomes  of  a  more  fiery  red ;  in  orange  or  yellow 

*  [A  certain  proportion  of  the  light  falling  upon  colored  bodies 
is  usually  sent  back  unchanged  by  superficial  reflection,  without 
undergoing  the  elective  absorption  to  which  the  color  of  the  sub- 
stance is  due.] 

28 


SCHBLLEN.]       SPECTRUM  ANALYSIS  EXPLAINED.  83 

light,  it  appears  orange  or  yellow,  but  deeper  in  tone ;  green 
rays  impart  to  it  something  of  their  own  tint,  but  as  the  red 
substance  can  reflect  only  a  few  of  the  green  rays,  it  appears 
pale  and  dull  by  their  light ;  it  seems  still  duller  and  darker 
in  blue  light,  and  with  indigo  and  violet  it  is  almost  black. 

These  phenomena  are  explained  by  the  supposition  that 
the  surfaces  of  colored  bodies  possess  the  property  of  re- 
flecting the  rays  of  one  particular  color  in  far  greater  pro- 
portion than  those  of  the  other  colors  ;  they  do  not  therefore 
appear  black  when  illuminated  by  a  light  differing  from  their 
own  natural  color.  Take,  for  example,  a  piece  of  paper 
half  of  which  is  colored  a  deep  blue  and  half  red :  the  col- 
ored rays  other  than  the  blue  and  red  are  not  all  absorbed : 
it  is  true  that  the  blue  piece  reflects  the  blue  rays  pre-emi- 
nently and  in  greatest  number,  as  the  red  part  does  the  red 
rays,  but  the  red  has  also  the  capability  of  reflecting  other 
rays  to  a  small  amount.  If  the  pure  yellow  light  of  a  spirit 
flame  impregnated  with  salt  be  allowed  to  fall  on  the  paper 
in  a  completely  dark  room,  the  paper  must  appear  black  if 
the  coloring  matter  reflect  only  the  red  and  blue  rays, 
because  the  yellow  rays  of  the  burning  sodium  will  be 
absorbed,  and  no  other  light  falls  upon  the  paper ;  but  this 
is  not  the  case.  The  paper  only  appears  black  on  the  blue 
part ;  the  red  half  is  still  visibly  colored,  though  of  a  de- 
cidedly yellow  shade.  We  therefore  conclude  that  the  blue 
of  the  paper  does  not  reflect  the  yellow  rays,  but  that  the  red 
has  that  power  in  a  small  degree.  Almost  all  colored 
objects  act  like  the  red  paper ;  they  reflect  pre-eminently 
one  particular  color,  namely,  that  one  of  which  they  appear 
by  daylight ;  but  they  are  able  also  to  reflect  in  small  quan- 
tities all  other,  or  at  least  some  other  colors,  and  so  they 
vary  in  tint  according  to  the  kind  of  light  in  which  they  are 
seen. 

The  colors  of  objects  are  very  rarely  pure  and  simple, 
like  those  of  the  spectrum  ;  most  of  them  are  composed  of 
several  colors,  and  can  be  decomposed  into  their  original 

29 


84  RECREATIONS    IN   POPULAR    SCIENCE.   [SCHELLE*. 

elements  by  a  prism.  As  without  prismatic  decomposition, 
we  are  unable  merely  from  the  color  of  an  object  to  say  pos- 
itively which  colors  are  absorbed  and  which  reflected,  so  it  is 
equally  impossible  for  us  to  decide,  from  the  color  of  a  flame, 
what  the  composition  of  its  light  may  be,  without  investiga- 
tion. The  light  of  the  sun,  the  lime-light,  the  magnesium 
light,  the  light  of  coal  gas,  petroleum,  and  oil,  all  appear  to 
us  more  or  less  white,  and  yet  the  spectra  of  the  various 
lights  differ  considerably.  It  is  true  they  all  contain  the 
whole  range  of  the  colors  of  the  spectrum,  from  red  to 
violet ;  but  each  color  is  present  in  very  different  proportions. 
The  light  from  gas,  oil,  and  candles  has  less  blue  than  that 
of  the  sun  and  the  lime-light,  and  very  much  less  violet.  A 
blue  material  will  therefore  reflect  less  blue  by  lamp,  gas,  or 
candle  light  than  by  daylight ;  the  color  will  not  only  be  flat 
and  dull,  but  will  have  a  touch  of  green  in  it,  on  account  of 
the  preponderance  of  yellow  light.  Blue  and  violet  espe- 
cially receive  a  green  tinge  by  candle  light,  in  which  these 
colors  appear  much  duller  than  in  daylight;  and  indeed 
sometimes,  according  to  the  nature  of  the  coloring  matter 
employed,  this  tint  is  so  decided  that  in  artificial  light  many 
kinds  of  green  cannot  be  distinguished  from  blue. 

ABSORPTION  OF  LIGHT  BY  SOLID  BODIES. 

By  the  term  absorption  we  have  already  designated  that 
action  by  which  light,  in  its  passage  through  certain  media, 
or  by  its  reflection  from  the  surfaces  of  bodies,  is  weakened, 
partially  retained,  or  entirely  stopped.  We  found  that  those 
substances  called  black  absorbed  rays  of  every  color,  and 
reflected  no  light  from  their  surfaces,  and  that  most  sub- 
stances absorbed  with  great  avidity  rays  of  certain  colors, 
while  they  were  insensitive  to  others.  The  cause  of  this 
absorption  is  probably  due  to  the  vibrations  of  the  ether 
being  communicated  to  the  ponderable  molecular  particles  of 
the  substance. 

30 


SCHELLEK.]      SPECTRUM  ANALYSIS   EXPLAINED.  85 

Similar  phenomena  are  noticed  when  light  is  transmitted 
through  colored  glass.  When  all  the  objects  in  a  landscape 
appear  red  through  a  red  glass,  it  is  because  the  glass  allows 
only  the  red  rays  to  pass  through,  and  absorbs  every  other 
colored  ray :  such  a  glass  is  transparent  only  to  red  light, 
and  is  opaque  to  every  other  color.  But  it  is  rarely  the  case 
that  colored  glass  is  transparent  for  one  color  only;  most 
kinds  of  glass  absorb  rays  of  certain  colors,  and  allow  the 
others  to  pass  through  in  very  different  proportions.  The 
naked  eye  is  unable  to  decide  which  of  the  colored  rays  are 
transmitted  through  a  colored  glass ;  this  can  only  be  accu- 
rately determined  by  analyzing  the  transmitted  light  by  a 
spectroscope  or  simple  prism. 

If  we  examine  by  a  spectroscope  the  transmitted  light  of 
the  colored  glass  that  we  before  made  use  of  for  obtaining 
red,  green,  and  blue  light,  it  will  at  once  be  seen  that  the 
ruby  red  glass  transmits  some  orange  and  even  some  yel- 
low rays,  as  well  as  the  red,  but  that  it  entirely  absorbs  the 
green,  blue,  and  violet  rays ;  the  cobalt  blue  glass  transmits 
some  violet  and  green  rays,  besides  the  blue,  but  absorbs  all 
the  red  rays.  If  both  glasses  be  laid  one  over  the  other,  and 
a  gas  flame  looked  at  through  them,  it  seems  as  if  scarcely  a 
single  ray  was  transmitted ;  the  red  glass  absorbing  the 
green,  blue,  and  violet  rays,  and  the  blue  glass  absorbing  the 
red  rays,  there  pass  through  only  traces  of  such  light  as  has 
not  been  entirely  absorbed,  and  this  causes  the  gas  flame  to 
appear  of  a  dull  yellow.  A  combination  of  several  glasses, 
or  indeed  any  single  glass  which  absorbs  all  the  colored  rays 
composing  white  light,  is  opaque,  that  is  to  say,  black ; 
glass  of  perfect  transparency,  absorbing  absolutely  none  of 
the  transmitted  light,  does  not  exist. 

3' 


86  RECREATIONS   IN  POPULAR   SCIENCE.    [SCHELLEM. 


RELATION  BETWEEN  THE  EMISSION  AND  THE  ABSORPTION 
OF  LIGHT. 

When  it  is  remembered  that  solid  bodies  in  a  state  of  in- 
candescence emit  a  much  greater  body  of  light  than  gases 
emit  in  a  similar  condition,  and  that  they  are  able  to  absorb 
a  much  greater  quantity  of  the  light  falling  on  them,  —  in 
certain  circumstances,  even  the  whole  of  it,  —  through  the 
transfer  of  the  ether  vibrations  to  their  ponderable  atoms ; 
when,  further,  it  is  remembered  that  just  those  substances 
\\\d(\.  give  out  heat  with  the  greatest  facility,  and  in  the  fullest 
quantity,  are  also  the  most  capable  of  receiving  heat  from 
without,  or  absorbing  it,  the  thought  is  suggested  that  there 
must  be  an  intimate  connection,  a  certain  reciprocity  between 
the  power  of  a  body  to  emit  light  (emission)  and  to  absorb 
it  (absorption).  That  the  temperature  of  the  substance  has 
an  influence  on  this  relation  between  its  emissive  and  ab- 
sorptive powers,  is  proved  by  the  phenomena  of  the  gas 
spectra  of  the  first,  second,  and  third  order,  as  well  as  by  the 
variety  of  absorption  spectra  exhibited  at  different  tempera- 
tures by  the  same  substance.  A  century  ago,  the  eminent 
mathematician  and  physicist,  Euler,  in  his  u  Theoria  lucis  et 
caloris,"  enunciated  the  principle  that  every  substance  ab- 
sorbs light  of  such  a  wave-length  as  coincides  with  the 
vibrations  of  its  smallest  particles.  Foucault  mentioned  in 
his  work  on  the  spectrum  of  the  electric  light,  published  in 
1849,  that  owing  to  the  impurity  of  the  carbon  points,  the 
intense  yellow  sodium  line  appeared,  and  was  changed  into  a 
black  line  when  sunlight  was  transmitted  though  the  electric 
arc.  Angstrom  gave  expression,  as  early  as  the  year  1853, 
to  the  general  law  that  a  gas,  when  luminous,  emits  rays  of 
light  of  the  same  refrangibility  as  those  'which  it  has 
power  to  absorb^  or,  in  other  words,  that  the  rays  which  a 
substance  absorbs  are  precisely  those  which  it  emits  when 
made  self-luminous. 

32 


SCHELLEN.]       SPECTRUM  ANALYSIS  EXPLAINED.  87 

But  all  these  facts  remained  isolated,  and  there  was  yet 
wanting  the  comprehensive  grasp  of  a  general  physical  law, 
under  which  the  individual  phenomena  could  be  arranged. 
It  was  reserved  to  Kirchhoff  to  discover  this  law,  and  to  es- 
tablish triumphantly  its  truth,  not  only  by  mathematical 
proof,  but  also,  in  many  striking  instances,  by  experiment. 

In  the  year  1860,  he  published  his  memoir  on  the  relation 
between  the  emissive  and  absorptive  powers  of  bodies  for 
heat,  as  well  as  for  light,  in  which  occurs  the  celebrated  sen- 
tence:  "The  relation  between  the  power  of  emission  and 
the  power  of  absorption  of  one  and  the  same  class  of  rays, 
is  the  same  for  all  bodies  at  the  same  temperature"  which 
will  ever  be  distinguished  as  announcing  one  of  the  most 
important  laws  of  nature,  and  which,  on  account  of  its  ex- 
tensive influence  and  universal  application,  will  render 
immortal  the  name  of  its  illustrious  discoverer. 


REVERSAL  OF  THE  SPECTRA  OF  GASES. 

From  KirchhofY's  law  it  follows  as  a  necessary  conse- 
quence that  gases  and  vapors,  in  transmitting  light,  absorb 
or  impair  precisely  those  rays  (colors)  which  they  them- 
selves emit,  when  rendered  luminous,  while  they  remain 
perfectly  transparent  to  all  other  colored  rays.  Luminous 
sodium  vapor,  for  example,  gives,  under  ordinary  circum- 
stances, a  spectrum  of  one  bright  yellow  double  line ;  it 
emits  therefore  this  yellow  light  only.  If  the  white  light  of 
the  sun,  the  electric  arc,  or  the  oxyhydrogen  lamp  be 
allowed  to  pass  through  the  vapor  of  sodium,  the  vapor  will 
abstract  or  extinguish  from  the  white  light  just  those  yellow 
rays  which  it  emitted  when  in  a  luminous  state.  While  the 
greater  part  of  these  yellow  rays  are  absorbed  by  the  sodium 
vapor,  all  the  other  rays  —  the  red,  orange,  green,  blue,  and 
violet  —  pass  through  unimpaired. 

The  important  result  of  these  investigations  is,  therefore, 
that  the  characteristic  bright  lines  of  sodium,  lithium,  etc., 

33 


88  RECREATIONS  IN  POPULAR  SCIENCE.     [SCHKLLBN. 

are  changed  into  dark  lines  when  the  intense  white  light  of 
incandescent  solid  or  liquid  bodies  passes  through  the  vapor 
of  'these  metals.  The  spectrum  of  luminous  sodium  vapor 
is  a  bright  yellow  (double)  line,  the  rest  of  the  field  in  the 
spectroscope  remaining  dark  ;  *  the  spectrum  of  an  incandes- 
cent solid  or  liquid  body,  after  it  has  passed  through  sodium 
vapor  at  a  lower  temperature  than  itself,  occupies  on  the 
contrary  the  whole  field  with  its  brilliant  colors,  excepting 
only  that  one  place  in  which  the  dark  sodium  line  is  found. 
As  therefore  the  bright  lines  of  gas  spectra  are  converted,  in 
these  experiments,  into  dark  lines,  while  the  dark  parts  of 
the  spectrum  are  changed  into  brilliant  colors  by  the  contin- 
uous spectrum  of  the  white  light,  the  entire  gas  spectrum 
seems  to  be  reversed  in  respect  of  its  illumination  :  for  this 
reason,  the  phenomenon  has  been  called,  after  Kirchhoff, 
"  the  reversal  of  the  spectrum" 

We  can  now  readily  predict  what  appearance  will  be  pre- 
sented in  the  spectroscope,  if  the  light  of  an  incandescent 
,  solid  or  liquid  body,  before  entering  the  slit  of  the  instru- 
ment, pass  through  a  less  highly  heated  atmosphere  of  any 
kind  of  vapor,  such  as  that  of  sodium,  lithium,  iron,  etc. 
The  incandescent  body  would  have  produced  a  continuous 
spectrum,  if  its  light  had  sustained  no  change  on  the  way ; 
but  in  the  vaporous  atmosphere  through  which  its  rays  must 
pass,  each  vapor  absorbs  just  those  rays  which  it  would  have 
emitted  if  luminous,  thereby  extinguishing  these  particular 
colors,  and  substituting  for  them  dark  bands  in  those  places 
of  the  continuous  spectrum  where  it  would  have  produced 
bright  lines.  The  spectroscope  shows  therefore  a  continuous 
spectrum,  extending  through  the  whole  range  of  colors, 
from  red  to  violet, .but  intersected  by  dark  lines;  the  sodium 
line,  the  two  lithium  lines,  the  numerous  iron  lines,  etc., 
appear  on  the  colored  ground  of  the  continuous  spectrum  as 
BO  many  dark  lines. 

*  See  No.  3  of  Frontispiece. 
34 


SCHELLEN.]       SPECTRUM  ANALYSIS   EXPLAINED.  89 

Spectra  of  this  kind  are  evidently  absorption  spectra; 
they  are  also  called  reversed  or  compound  spectra.  If  a 
complete  coincidence  can  be  established  in  such  a  spectrum, 
by  means  of  either  a  prism  of  comparison,  or  a  scale,  be- 
tween the  characteristic  bright  lines  of  the  gas  spectrum  of 
a  certain  substance,  with  the  same  number  of  dark  lines, 
the  conclusion  may  be  admitted  that  in  the  absorptive  atmos- 
phere which  has  produced  the  dark  lines,  the  same  substance 
is  contained  in  a  condition  of  vapor. 

THE  SOLAR  SPECTRUM  AND  THE  FRAUNHOFER  LINES. 

The  most  brilliant  example  of  a  reversed  spectrum,  —  that 
is  to  say,  a  continuous  spectrum,  crossed  by  dark  absorption 
lines,  —  is  afforded  by  the  sun.  If  an  ordinary  spectroscope, 
armed  with  a  telescope  of  low  power,  be  directed  to  a  bright 
sky,  with  a  rather  wide  opening  of  the  slit,  a  magnificent 
continuous  spectrum  will  be  seen,  exhibiting  the  most  beauti- 
ful and  brilliant  colors,  without  either  bright  or  dark  lines. 
But  if  the  slit  be  narrowed  so  as  to  obtain  the  purest  possible 
spectrum,  and  the  focus  of  the  telescope  be  very  accurately 
adjusted,  the  spectrum,  now  much  fainter,  will  be  seen  to  be 
crossed  by  a  number  of  dark  lines  and  cloudy  bands.  If,  by 
the  use  of  several  prisms,  the  spectrum  be  lengthened,  and  a 
higher  magnifying  power  employed,  these  thick  lines  and 
bands  will  become  resolved  into  separate  fine  lines  and 
groups  of  lines,  which  are  so  sharply  defined  and  so  charac- 
teristically grouped  that,  by  the  help  of  a  scale,  they  are 
easily  impressed  upon  the  memory,  and  distinguished  one 
from  another. 

As  early  as  1802,  these  dark  lines  in  the  solar  spectrum 
had  been  observed  and  described  by  Wollaston ;  later,  in 
1814,  they  were  more  carefully  examined  and  mapped  by 
Fraunhofer,  of  Munich  ;  and  later  still  by  Becquerel,  Zante- 
deschi,  Matthiessen,  Brewster,  Gladstone,  and  others;  but 
their  origin  and  nature  remained  a  mystery,  notwithstanding 

35 


90  RECREATIONS   IN   POPULAR   SCIENCE.    [SCHELLEN. 

the  acutest  reasoning  and  most  painstaking  researches  of 
many  able  physicists,  until  Kirchhoff  made  his  splendid 
discovery  in  1859. 

Fraunhofer  was  able  to  distinguish  with  certainty  about 
six  hundred  lines ;  he  found  also  that  with  the  same  prism 
and  telescope  they  always  kept  the  same  relative  order  and 
position,  and  were  therefore  peculiarly  adapted  to  serve  as 
marks  for  denoting  the  place  of  any  single  set  of  colored 
rays,  and  for  determining  the  refrangibility  of  any  particular 
cplojvx 

^^To  facilitate  reference  to  any  of  the  innumerable  colors  of 
the  solar  spectrum  (Frontispiece,  No.  i),  Fraunhofer,  whose 
drawing  is  accurately  represented  in  -F-igriV-.,  selected  out 
of  the  great  number  he  observed  eight  characteristic  lines, 
situated  in  the  most  important  places  of  the  spectrum,  which 
he  designated  by  the  letters  A,  B,  C,  D,  E,  F,  G,  H ;  of 
these  lines  A  and  B  lie  in  the  red,  C  in  the  red  near  the 
orange,  D  in  the  orange,  forming  a  double  line  with  a  high 
power,  E  in  the  yellow,  F  on  the  borders  of  the  green  and 
blue,  G  in  the  dark  blue  or  indigo,  and  H  in  the  violet.  Be- 
sides these  lines,  there  is  a  noticeable  group  #,  of  fine  lines 
between  A  and  B,  and  also  a  group  £,  consisting  of  three 
fine  lines,  between  E  and  F.  It  was  remarked  even  by 
Fraunhofer  that  the  position  of  the  two  dark  lines  in  the 
solar  spectrum  designated  by  him  D,  were  coincident  with 
the  two  bright  lines  shown  by  the  light  of  a  lamp,  now 
known  as  the  double  sodium  line.  These  dark  lines  of  the 
solar  spectrum  have  been  called,  after  their  discoverer,  the 
Fraunhofer  lines. 

BInce^*raunHoler  counted  in  the  spectrum  more  than  six 
hundred  dark  lines,  Brewster  has  counted  two  thousand,  and 
others,  by  causing  the  refracted  rays  to  pass  successively 
through  several  prisms  (some  using  as  many  as  nine  at  one 
observation),  the  existence  of  three  thousand  lines  has  been 
ascertained.  These  lines  have  been  carefully  mapped  by 
Angstrom  and  Thalen,  and  Kirchhoff,  who  has  carefully 

36 


SCHELLEN.]       SPECTRUM   ANALYSIS    EXPLAINED. 


91 


studied  them,  and  ascertained 
wherein  they  coincide  with  the 
bright  lines  of  metals  known 
to  us,  says,  "  The  most  strik- 
ing coincidences  between  the 
spectrum  lines  of  terrestrial 
elements  and  the  dark  lines  of 
the  solar  spectrum  are  shown 
m  iron,  sodium,  potassium, 
calcium,  magnesium,  manga- 
nese, chromium,  nickel,  and  hy- 
drogen ;  the  spectrum  lines  of 
these  substances  not  only  agree 
exactly  with  the  dark  lines  in 
position  and  breadth,  but  pro- 
claim their  relationship  to  them 
by  a  similar  degree  of  inten- 
sity. The  brighter,  for  in- 
stance, a  spectrum  line  ap- 
pears, so  much  the  darker  will 
its  corresponding  line  be  in  the 
solar  spectrum. 

Fig.  IV.  shows  the  principal 
Fraunhofer  lines,  A,  B,  C,  etc., 
and  many  intermediate  ones. 
It  also  shows  the  coincidence 
between  sixty-five  of  the  bright 
lines  in  the  spectrum  of  iron 
and  the  same  number  of  dark 
lines  in  the  solar  spectrum, 
proving  beyond  a  doubt  the 
existence  of  iron  in  the  sun. 
Other  observations  prove  as 
conclusively  the  existence  of 
sodium,  potassium,  and  the 
other  metals  which  are  above 
mentioned. 

37 


FIG.  IV. 


92  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLEM 

THE  TELLURIC  LINES. 

In  addition  to  the  Fraunhofer  lines,  there  are  other  dark 
lines  in  the  solar  spectrum,  which  are  called  atmospheric  or 
telluric  lines.  These  are  variable,  and  are  caused  by  the 
varying  condition  of  the  earth's  atmosphere.  Janssen  found 
them  to  be  darkest  at  sunrise  and  sunset,  and  less  intense  in 
the  middle  of  the  day,  a  periodicity  of  change  which  at 
once  proves  their  atmospheric  origin,  but  they  were  never 
entirely  absent  from  the  spectrum.  He  instituted  a  series  of 
experiments  by  which  he  found  that  a  large  number  of  the 
variable  lines  in  the  solar  spectrum  are  due  to  the  presence 
of  aqueous  vapor  in  the  earth's  atmosphere,  and  also  a 
method  secured  for  detecting  the  presence  of  aqueous  vapor 
in  the  heavenly  bodies. 

Angstrom  says  that  nearly  all  the  changes  of  color  ob- 
served in  the  red  glow  of  sunrise  and  sunset  find  a  simple 
explanation  in  the  phenomena  of  atmospheric  absorption, 
whereby  all  the  ingenious  and  elaborate  explanations  hith- 
erto attempted  are  completely  set  aside. 

It  is  fully  admitted  that  other  heavenly  bodies  besides  the 
earth  maybe  surrounded  by  an  atmosphere;  Janssen's  dis- 
covery of  the  spectrum  of  aqueous  vapor  furnishes  the 
means  of  ascertaining  whether  this  vapor,  indispensable  to 
the  maintenance  of  all  the  living  organisms  «f  our  planet,  is 
also  present  in  the  other  celestial  bodies.  Repeated  obser- 
vations undertaken  by  Janssen  on  the  high  mountains  of 
Italy  and  Greece  have  already  furnished  proof  that  aqueous 
vapor  is  present  in  the  atmospheres  of  the  planets  Mars  and 
Saturn. 

.SPECTRUM  APPARATUS. 

The  thought  is  perhaps  rising  in  the  minds  of  many  who 
have  accompanied  us  thus  far,  that  ithe  production  of  the 
spectrum  of  a  substance  .for  the  purposes  of  analytical  ex- 
amination is  encumbered  with  great  difficulties  and  many 
troublesome  details,  involving  too  much  labor  to  be  available 

38 


SCHELLEN.J        SPECTRUM  ANALYSIS  EXPLAINED.  93 

for  the  use  of  the  chemist  and  the  physicist.  This  is,  how- 
ever, not  the  case  ;  if,  iu  our  mode  of  illustration,  a  powerful 
galvanic  battery,  and  the  electric  lamp,  with  its  revolving 
table  and  large  screen,  have  been  employed,  it  has  been  only 
to  show  how,  by  the  extraordinary  heat  and  light  of  the 
voltaic  arc,  the  simple  phenomena  on  which  spectrum  anal- 
ysis is  based  can  be  made  visible  to  many  hundred  spectators 
at  once  in  a  large  lecture-room.  When,  however,  the  light 
from  the  heated  vapors  need  not  be  greater  than  is  required 
for  a  single  observer,  the  whole  electric  apparatus  may  be 
dispensed  with,  and  the  simple  Bunsen  burner  substituted ; 
in  place  of  the  large  screen  of  paper  that  reflected  the  light, 
the  small,  sensitive  screen  of  nerves  —  the  retina  of  the  hu- 
man eye  —  becomes  the  surface  on  which  the  spectrum  is 
received  ;  and  the  whole  cumbrous  contrivance  occupying  so 
much  space  is  replaced  by  a  small  spectrum  apparatus  as 
trustworthy  as  it  is  easy  to  manipulate. 

FIG.  V. 


THE   COMPOUND   SPECTROSCOPE. 

Fig.  V.  shows  a  compound  spectroscope.  With  this  we 
are  enabled  to  use  two  flames,  and  the  apparatus  is  so  ar- 
ranged that  we  can  see  the  two  spectra,  one  above  the  other, 
and  compare  them  with  each  other.  For  instance,  putting  a 
small  quantity  of  the  substance  we  know  to  contain  sodium 

39 


94  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLBN. 

into  one  flame,  we  place  a  substance  supposed  to  contain 
sodium  in  the  other  flame,  and  then,  by  means  of  a  small 
reflecting  prism  placed  at  the  end  of  the  slit,  we  have  the 
spectra  of  both  sent  into  the  telescope,  one  above  the  other, 
and  can  tell  at  once  whether  the  lines  coincide  with  each 
other.  Another  arrangement  for  facilitating  the  comparison 
of  spectra  consists  of  the  illuminated  millimetre  scale  con- 
tained in  the  tube  g  (Fig.  V.),  a  magnified  reflection  of 
which  is  thrown  into  the  telescope^",  from  the  surface  of  the 
prism  a.  The  scale  is  thus  seen  between  the  two  spectra, 
and  the  position  of  any  line  can  be  accurately  measured,  and 
ascertained. 

The  reader  is  now  in  a  position  to  understand  the  use  of 
the  various  parts  of  a  complete  spectrum  apparatus,  espe- 
cially the  three  tubes  directed  to  the  prism  at  different  an- 
gles, as  in  that  constructed  by  Kirchhoff  and  Bunsen.  The 
eye  of  the  observer  is  placed  in  the  axis  of  the  telescope 
directed  to  that  surface  from  which  the  light  emerges  in  the 
form  of  a  spectrum  ;  the  opposite  surface  of  the  prism  re- 
ceives through  the  slit  and  collimating  lens  the  light  emitted 
from  the  object  to  be  examined  ;  at  the  side  of  the  observer 
is  the  tube  carrying  the  illuminated  scale,  or  the  micrometer 
screw,  so  that  the  mark  coinciding  with  any  division  of  the 
scale  may  be  placed  on  any  line  of  the  spectrum. 

CONCLUSION. 

If  we  have  given  our  readers  anything  approaching  an 
adequate  idea  of  the  beauty  and  utility  of  this  wonderful 
discovery,  we  feel  sure  it  will  only  stimulate  them  to  inves- 
tigate and  study  it  further.  It  was  our  intention  to  have 
continued  the  subject,  and  closed  this  article  with  a  resume 
of  the  discoveries  made  by  means  of  spectrum  analysis,  but 
we  find  it  impossible  to  condense  the  matter  sufficiently  to 
bring  it  into  a  size  appropriate  to  these  papers,  and  we  defer 
it  to  the  next  number,  which  will  be  devoted  to  Spectrum 
Analysts  Discoveries. 

40 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  95 

i 


5,    Sputum   Analysis 

Showing  its  Application  in  Microscopical  Research,  and  to  Discov- 
eries of  the  Physical  Constitution  and  Movements  of 

THE  HEAVENLY  BODIES. 

IN  the  preceding  article  we  gave  our  readers,  in  general 
terms,  the  scope  of  this  great  discovery,  and  its  impor- 
tance to  the  scientific  world,  and,  to  make  it  plainer,  laid 
down  in  a  condensed  form  the  received  laws  of  the  theory  of 
sound,  heat,  light,  and  color,  a&  well  as  an  explanation  of 
the  modes  in  use  of  detecting  minute  particles  of  elements, 
where  all  other  means  had  failed.  We  also  showed  that,  by 
means  of  it,  many  substances,  before  unknown,  had  been 
discovered ;  and  it  is  not  doubted  that  others  will  be  in  the 
future,  as  the  discovery  is  still  in  its  infancy. 

THE  MICROSPECTROSCOPE. 

In  addition  to  ordinary  spectroscopes,  Messrs.  Sorby  and 
Browning  have  devised  a  combination  of  the  microscope 
and  spectroscope,  called  the  microspectroscope,  which  ren- 
ders it  possible  to  examine  very  minute  traces  of  substances. 
This  application  of  the  spectroscope  has  been  very  useful  in 
investigating  substances  which  have  special  importance  in 
physiology  and  pathology:  thus  in  examining  normal  and 
diseased  blood,  in  detecting  albumen  in  urine,  and  in  ascer- 
taining the  rate  at  which  certain  substances  pass  into  the 
various  fluids  of  the  system,  as  well  as  investigating  sup- 
posed cases  of  poisoning.  The  characteristic  absorption 
bands  which  certain  liquids,  such  as  wine,  beer,  etc.,  present 
in  their  normal  state,  compared  with  those  yielded  by  adul- 
terated substances,  furnish  a  delicate  and  certain  means  of 
detecting  the  latter. 


96  RECREATIONS   IN   POPULAR   SCIENCE.   [SCHELLBN. 

An  instance  of  its  use  in  detecting  the  cause  of  impurity  in 
water  is  related,  as  follows :  The  water  used  by  the  inhab- 
itants of  a  crowded  court,  amongst  whom  several  cases  of 
typhoid  fever  had  appeared,  was  drawn  from  a  rather  shal- 
low well,  and  was  highly  charged  with  various  unoxidized 
compounds  of  nitrogen.  It  was  suspected  that,  from  some 
defect  of  drainage,  the  contents  of  a  public  urinal  obtained 
entrance  to  the  well.  The  fact  that  the  well-water  contained 
seven  times  as  much  common  salt  as  the  normal  water  of 
the  vicinity,  was  some  confirmation  of  the  suspicion.  Pro- 
fessor Church  obtained  absolute  proof  'of  the  fact  by  the  fol- 
lowing method.  He  introduced  two  grammes  of  a  lithium 
salt  into  the  urinal,  and  two  hours  later  was  enabled  readily 
to  detect  with  the  spectroscope  the  presence  of  lithium  in  a 
litre  of  the  well-water,  which  by  previous  examination  had 
shown  no  traces  of  this  substance.  Many  other  instances  of 
its  use  for  similar  purposes  might  be  cited. 

Spectrum  analysis  has  thus  opened  a  wide  field  of  investi- 
gation to  the  physiologist,  the  physician,  the  botanist,  the 
zoologist,  the  chemist,  and  the  technologist,  and  the  labors 
undertaken  in  these  various  departments  of  science  have 
already  yielded  valuable  results. 

It  was  shown  in  our  last  paper  that  by  it  we  have  at  last 
the  means  of  forming  a  definite  idea  of  the  physical  consti- 
tution of  the  heavenly  bodies,  and  ascertaining  the  existence 
of  atmospheres  around  other  planets,  and  other  means  of 
supporting  life  such  as  exist  on  this  earth.  We  will  proceed 
to  give  a  resume  of  the  other  discoveries  regarding  the  phys- 
ical constitution  and  movements  of  the  heavenly  bodies. 

KIRCHHOFF'S  THEORY  OF  THE   PHYSICAL   CONSTITUTION 
OF  THE  SUN. 

It  had  long  been  assumed  that  the  gaps  in  the  colors  of 
the  solar  spectrum  which  form  the  Fraunhofer  dark  lines, 
were  due  to  an  absorption  of  the  corresponding  colored  rays 

2 


SCHEILEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  97 

in  the  atmosphere  of  the  sun ;  but  no  explanation  could  be 
given  of  this  phenomenon.  The  cause  of  this  absorption 
was  ascertained  by  Kirchhoff  in  his  discovery  that  a  vapor 
absorbs  from  white  light  just  those  rays  which  it  emits  when 
luminous,  and  he  proved  the  whole  system  of  the  Fraunhofer 
lines  to  be  mainly  produced  by  the  overlying  of  the  reversed 
spectra  of  such  substances  as  are  to  be  found  in  the  earth. 
He  thus  arrived  at  a  new  conception  of  the  physical  consti- 
tution of  the  sun  which  is  entirely  opposed  to  the  theories 
held  by  Wilson  and  Sir  William  Herschel  in  explanation  of 
the  solar  spots. 

According  to  Kirchhoff,  the  sun  consists  of  a  solid  or  par- 
tially liquid  nucleus  in  the  highest  state  of  incandescence, 
which  emits,  like  all  incandescent  solid  or  liquid  bodies, 
every  possible  kind  of  light,  and  therefore  would  of  itself 
give  a  continuous  spectrum  without  any  dark  lines.  This 
incandescent  central  nucleus  is  surrounded  by  an  atmospher3 
of  lower  temperature,  containing,  on  account  of  the  extreme 
heat  of  the  nucleus,  the  vapors  of  many  of  the  substances 
of  which  this  body  is  composed.  The  rays  of  light,  there- 
fore, emitted  by  the  nucleus,  must  pass  through  this  atmos- 
phere before  reaching  the  earth,  and  each  vapor  extinguishes 
from  the  white  light  those  rays  which  it  would  itself  emit  in 
a  glowing  state.  Now  it  is  found,  when  the  sun's  light  is 
analyzed  by  a  prism,  that  a  multitude  of  rays  are  extin- 
guished, and  just  those  rays  which  would  be  emitted  by  the 
vapors  of  sodium,  iron,  calcium,  magnesium,  etc.,  were  they 
made  self-luminous;  consequently  the  vapors  of  the  fol- 
lowing substances,  sodium,  iron,  potassium,  calcium,  barium, 
magnesium,  manganese,  titanium,  chromium,  nickel,  cobalt, 
hydrogen,  and  probably,  also,  zinc,  copper,  and  gold,  must 
exist  in  the  solar  atmosphere,  and  these  metals,  therefore, 
must  also  be  present  to  a  considerable  extent  in  the  body  of 
the  sun. 

Could  the  light  from  the  sun's  nucleus  in  any  way  be  set 
aside,  and  only  that  of  the  incandescent  vapors  of  the  sun's 

3 


98  RECREATIONS   IN  POPULAR  SCIENCE.   [SCHELLEN. 

atmosphere  be  received  through  the  slit  of  the  spectroscope, 
a  spectrum  would  then  be  obtained  composed  of  the  actual 
spectra  of  these  substances,  that  is  to  say,  the  same  system 
of  bright  colored  lines  which  now  appear  as  the  dark  Fraun- 
hofer  lines. 

THE  SOLAR  SPOTS  ;  THE  FACUL^E  AND  THEIR  SPECTRA. 

It  would  lead  us  too  far  from  our  subject  were  we  to  dwell 
upon  the  phenomena  of  the  solar  spots,  important  as  they 
are  for  acquiring  a  knowledge  of  the  physical  constitution 
of  the  sun,  or  enter  upon  a  full  description  of  their  form, 
their  mode  of  formation  and  disappearance,  their  motion, 
their  connection  with  the  sun's  rotation  upon  its  axis,  their 
periodic  occurrence,  and  the  various  hypotheses  that  have 
been  formed  as  to  their  nature ;  but,  on  the  other  hand,  we 
must  still  less  be  silent  on  the  subject,  since  spectrum  anal- 
ysis has  investigated  these  wonderful  appearances  with  a 
success  which  has  added  much  to  our  knowledge  of  the 
constitution  of  the  sun. 

Fig.  6  shows  a  remarkable  group  of  solar  spots.  These, 
and  indeed  a  large  proportion  of  solar  spots,  consist  princi- 
pally of  a  dark,  almost  black,  central  portion,  the  umbra  * 
surrounded  by  a  space  somewhat  less  dark  called  the  pen- 
umbra: the  umbra  has  generally  an  irregular  form,  while 
the  penumbra  exhibits  a  structure  radiating  towards  the 
centre. 

If  the  sun  be  observed  with  a  high  power,  the  surface  pre- 
sents by  no  means  a  uniform  appearance ;  a  multitude  of 
bright  and  dark  stripes  cross  each  other  in  all  directions,  and 

*  The  dark  central  part  of  a  spot  has  been  distinguished  through- 
out by  the  name  umbra,  in  accordance  with  the  usual  custom  of  as- 
tronomers. Mr.  Dawes  showed  that  within  this  part  of  a  spot  one 
or  more  darker  spots  may  generally  be  observed,  to  which  he  gave 
the  name  of  nucleus. 

4 


FIG.  VI. 


fi/tt&»'%<5^«9KTCT\«tS&&' 


Group  of  Solar  Spots  observed  and  drawn  by  Nasmyth,  5th  June,  1864. 


TOO  RECREATIONS    IN  POPULAR    SCIENCE.    [SCHELLEN. 

the  luminous  surface  appears  like  a  net  of  bright  meshes 
interwoven  with  dark  threads  and  small  dark  pores.  Th- 
brightest  portions  (Fig.  6)  show  a  more  or  less  elongated 
form,  which  suggested  to  Nasmyth  the  name  of  "  willow 
leaves,"  while  Dawes  compares  them  to  "  bits  of  straw," 
and  Huggins  calls  them  merely  "  granules." 

On  this  uneven  and  ever-varying  bright  background  the 
spots  make  their  appearance  in  the  greatest  variety  of  form 
and  size.  The  penumbra  not  unfrequently  stretches  across 
the  black  central  portion  in  various  places  (see  right  hand 
spot,  Fig.  6),  and  generally  appears  much  darker  at  the 
outer  edges,  where  the  spot  touches  the  bright  part  of  the 
sun's  surface,  than  in  other  places.  Very  often  the  penum- 
bra is  traversed  by  few  or  more  bright  curved  bands,  stretch- 
ing from  the  outer  edge  towards  the  nucleus,  generally  at 
right  angles  to  the  confines  of  the  nucleus  and  penumbra, 
which  give  the  spot  the  appearance  as  if  a  number  of  streams 
of  some  luminous  matter  had  broken  through  the  dam  formed 
by  the  penumbra,  to  fall  into  the  abyss  of  the  umbra.  (See 
central  spot,  Fig.  6.)  Even  the  umbra  itself  is  often  crossed 
by  one  or  more  broad  luminous  bands  called  bridges,  by 
which  it  is  divided  into  several  portions. 

Besides  the  dark  spots,  and  chiefly  in  their  immediate 
neighborhood,  bright  places  make  their  appearance  on  the 
sun's  surface,  which  have  been  called  faculcz.  They  are 
generally  the  attendants  of  solar  spots,  and  are  especially  to 
be  seen  at  the  extreme  edge  of  the  penumbra  when  the  spot 
has  reached  the  sun's  limb,  that  is,  its  edge  or  border ;  that 
they  are  not  the  effect  of  contrast  between  the  dark  spot  and 
the  neighboring  brightness,  is  proved  by  the  circumstance 
that  every  spot  is  not  accompanied  by  fuculae,  and  that  very 
frequently  isolated  faculae  are  to  be  seen,  which  are  almost 
always  the  precursor  of  a  coming  spot. 

The  faculae, '  like  the  spots,  vary  considerably  in  form ; 
generally  they  are  round  and  concentrated,  but  often  they 

6 


SCHELLEN.]      SPECTRUM  ANALYSIS  DISCOVERIES.  IOI 

have  the  appearance  of  long  stripes  of  light,  disposed  like 
veins,  converging  from  all  sides  towards  a  spot. 

The  wreathed  faculae  are  almost  always  followed  in  a  few 
days  by  the  appearance  of  a  group  of  spots ;  among  the 
vein-like  waves  of  light  visible  in  many  places,  more  espe- 
cially towards  the  sun's  limb,  there  is  first  developed  a  dull 
scar-like  place  out  of  which  the  spots  are  formed,  sometimes 
singly,  and  sometimes  in  groups ;  and  not  unfrequently  the 
formation  of  a  spot  may  be  predicted  from  the  increased 
intensity  of  light  at  that  place  on  the  sun's  disk. 

When  a  spot  is  observed  near  the  sun's  limb  or  edge  in 
the  midst  of  the  surrounding  faculae,  it  is  difficult  to  avoid 
the  impression  that  the  spot  lies  in  a  hollow  between  bright 
overhanging  mountains ;  and  it  was  observed  by  Secchi  on 
the  5th  of  August,  1865,  that  the  faculae,  when  they  reached 
the  western  limb  of  the  sun,  appeared  like  small  projections 
and  irregularities  upon  the  sharply  defined  limb  of  the  sun. 
4  Although  the  real  connection  between  the  faculae  and  the 
spots  is  not  yet  fully  understood,  it  may  be  safely  concluded 
from  these  observations  that  the  spots  lie  deeper  in  the  solar 
surface  than  do  the  faculae,  and  that  these  faculae  are  moun- 
tainous elevations  of  the  luminous  matter  forming  the  photo- 
sphere, by  which  the  spot  is  surrounded  in  a  wide  circuit  as 
by  a  wall. 

The  group  of  solar  spots  observed  and  drawn  by  Nasmyth 
on  the  5th  of  June,  1864,  is  given  in  Fig.  6,  in  which  all  the 
details  characteristic  of  a  spot  are  to  be  recognized  —  the 
black  umbra,  the  penumbra  in  a  variety  of  forms,  composed 
of  the  4k  leaves"  directed  towards  the  umbra,  and  the  sur- 
rounding luminous  surface  of  the  sun  presenting  its  usual 
granulated  appearance.  This  surface  is  called  the  photo- 
sphere, a  name  given  without  reference  to  any  particular 
theory  as  to  its  physical  constitution  or  structure.  The  pho- 
tosphere is  entirely  covered  with  pores,  or  small  spots,  less 
luminous  than  the  other  parts ;  where  they  congregate,  and 
become  conspicuous  by  forming  a  black  umbra  and  shaded 

7 


102  RECREATIONS    IN    POPULAR    SCIENCE.  [SCHELLEK. 

penumbra,  they  constitute  the  ordinary  solar  spot;  where 
the  portions  of  greater  brilliancy  than  the  surrounding  parts 
of  the  photosphere  congregate,  they  form  the  faculce,  and 
these  generally  accompany  the  spots,  or  precede  their  for- 
mation. 

If  a  solar  spot  be  watched  in  the  telescope  from  day  to 
day,  or  from  hour  to  hour,  it  will  soon  be  seen  to  change  in 
form ;  it  increases  or  diminishes,  or  completely  vanishes 
away,  while  new  spots  make  their  appearance.  In  the 
process  of  disappearing,  the  dark  umbra  first  gradually 
contracts  until  it  becomes  invisible,  leaving  the  dusky  pen- 
umbra perceptible  for  some  time  longer.  Not  unfrequently 
a  spot  breaks  up  into  several  spots,  and  occasionally  a  group 
unites  to  form  one,  and  sometimes,  even,  one  spot  is  seen  to 
pass  over  another,  partially  covering  it,  and  then  with- 
drawing from  it.  In  all  these  changes,  the  spots  exhibit  an 
amount  of  mobility  displayed  in  general  only  by  liquid  or 
vaporous  masses. 

The  formation  and  changes  in  the  configuration  of  a  spot 
may  often  be  watched  during  the  course  of  observation,  and 
it  not  unfrequently  happens  that  the  appearance  of  a  group 
of  spots  is  so  entirely  changed  from  one  day;to  another,  that 
it  can  no  longer  be  recognized  in  the  new  form  it  has  as- 
sumed. 

'On  the  other  hand,  there  are  spots  presenting  scarcely  any 
change,  which  preserve  nearly  the  same  form  for  many  days 
together.  Spots  of  this  kind  are  of  the  highest  value  to  the 
astronomer,  as  they  afford  the  only  means  of  ascertaining 
the  time  of  the  revolution  of  the-sun  upon  its  axis,  the  posi- 
tion of  this  axis,  and  its  inclination  to  the  earth's  orbit. 

It  not  unfrequently  happens,  that 'the  same  spot  which  has 
been  observed  to  disappear  on  the  western  limb  has,  in  the 
course  of  about  fourteen  days,  been  seen  to  reappear  ort  the 
eastern  Hrnb,  and  in  the  lapse  of  another  fourteen  days  has 
.disappeared  a  second  time  on  the  western  limb  —  a  phe- 
nomenon tliat  proves  beyond  a  doubt  that  the  spots  are  con- 

8 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  103 

nected  with  the  surface  of  the  sun,  and  that  the  sun  itself 
has  a  revolution  upon  its  axis.  If  the  time  required  for  the 
earth's  motion  round  the  sun  be  allowed  for  in  this  revolution 
of  the  spot,  the  result  will  show,  according  to  Sporer,  a 
mean  time  of  rotation  for  the  sun  amounting  to  twenty-five 
days,  five  hours,  thirty-eight  minutes. 

KirchhofT  considers  these  forms  to  be  cloud-like  condensa- 
tions in  the  sun's  atmosphere,  which  are  produced  by  the 
loss  of  the  solar  heat  by  radiation,  in  the  same  way  as  the 
aqueous  vapors  of  the  earth's  atmosphere  are  formed  into 
mist  and  cloud.  When  such  clouds  arise  over  the  bright 
and  glowing  surface  of  the  sun,  they  obscure  the  light  of 
the  sun  at  that  spot,  and  it  is  but  natural  that  these  cloudy 
masses,  so  irregularly  formed,  should  also  become  further 
condensed,  or  be  dispersed  with  the  same  amount  of  irreg- 
ularity, according  as  they  come  in  contact  with  cooler  or 
warmer  streams  of  gas. 

The  results  of  the  spectrum  observations  of  Secchi,  Lock- 
yer,  and  Young,  important  and  valuable  as  they  are,  remain 
as  yet  too  isolated  and  unconnected  with  telescopic  observa- 
tions of  the  spots  and  faculae  to  yield  material  sufficient  for 
explaining  the  nature  of  these  forms.  This  much,  however, 
may  be  regarded  as  certain  —  that  the  phenomena  of  the  in- 
crease in  the  width  and  intensity  of  the  Fraunhofer  lines,  as 
well  as  the  appearance  of  new  dark  bands  in  the  spectrum 
of  the  umbra,  are  produced  by  the  increased  absorptive 
power  exercised  by  the  substances  of  'which  the  spot  is 
formed. 

When  the  white  light  of  the  sun's  nucleus  which  has 
already  suffered  absorption  from  the  absorptive  stratum 
passes  through  the  vaporous  matter  of  a  spot,  it  undergoes  a 
yet  further  absorption  from  the  additional  matter  which  the 
spot  contains.  As,  therefore,  the  lines  of  calcium  and  iron 
are  considerably  affected  in  the  spectrum  of  a  spot,  the  so- 
dium lines  in  a  smaller  degree,  and  to  some  extent  those  of 
magnesium,  it  may  be  concluded  that  the  substance  forming 

9 


104  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLEN. 

the  solar  spots  is  composed  pre-eminently  of  vapors  of  cal- 
cium, iron,  titanium,  sodium,  barium,  and  magnesium,  and 
that  these  substances  occur  in  layers  of  varying  thickness, 
and  in  very  different  proportions. 

That  hydrogen  gas  constitutes  an  important  element  in 
the  formation  of  the  spots,  is  shown  in  the  most  unequivocal 
manner  by  the  spectrum.  The  hydrogen  lines  are  most 
affected  in  the  parts  that  lie  close  to  the  umbra,  in  the  bridge, 
when  one  is  formed,  and  in  the  penumbra. 

An  explanation  of  this  phenomenon  is,  that  hydrogen  gas 
breaks  forth,  from  time  to  time,  from  the  interior  of  the  in- 
candescent solar  nucleus.  Owing  to  its  extreme  lightness, 
this  gas  would  rise  in  enormous  pillars  of  flame  (promi- 
nences) over  the  absorptive  vaporous  stratum  of  the  photo- 
sphere, and,  in  consequence  of  the  cooling  ensuing  from  ex- 
pansion, would  enter  into  a  variety  of  chemical  combina- 
tions, especially  with  oxygen ;  the  uncombined  part  would 
then  flow  to  the  side,  while  that  in  combination  with  oxygen 
(steam)  and  the  other  solar  substances  would  form  gaseous  or 
vaporous  masses,  which,  from  their  nature  as  well  as  from 
their  continued  cooling,  would  be  heavier  than  the  hydrogen 
gas,  and  would  sink  down  from  their  greater  gravity.  It  is  to 
be  expected  that  the  stream  of  gas  on  rising  would  carry  up 
with  it  a  quantity  of  those  substances  that  exist  in  the  sun's 
nucleus  and  the  surounding  stratum  of  absorptive  vapor  (the 
photosphere)  ;  if  these  substances,  themselves  incandescent, 
were  present  in  sufficient  quantities  in  the  luminous  hydro- 
gen gas,  their  characteristic  lines  would  be  seen  as  bright 
lines  in  the  spectrum  of  the  pillars  of  flame.  During  the 
recent  total  eclipses,  many  such  lines  were  in  fact  observed, 
together  with  the  bright  hydrogen  lines,  in  the  prominences, 
a  description  of  which  will  be  given  farther  on ;  they  can 
now  be  observed  daily,  sometimes  in  great  numbers,  upon 
the  sun's  disk. 

When  the  force  of  the  gas  eruption  has  somewhat  subsided, 
and  the  chemical  combinations  ensue,  producing  vaporous 

10 


SCHELLEN.J      SPECTRUM  ANALYSIS  DISCOVERIES.  105 

precipitations  of  many  kinds,  the  formation  of  the  spot  be- 
gins. The  heavier  portions  of  these  precipitations  sink 
down,  and  form  the  umbra  of  a  spot  at  the  place  of  greatest 
condensation,  while  the  parts  which  are  less  dense  constitute 
the  penumbra..  The  vaporous  umbra,  however,  though 
apparently  quite  black,  is  yet  able  to  transmit  a  considerable 
amount  of  sunlight ;  indeed,  according  to  Zollner's  meas- 
urements, the  black  umbra  of  a  spot  emits  fottr  thousand 
times  as  much  light  as  that  derived  from  an  equal  area  of 
the  full  moon.  This  statement  is  fully  confirmed  by  the 
results  of  spectrum'  analysis,  for  even  the  blackest  umbra 
yields  a  spectrum  exhibiting  all  the  details  of  full  sunlight. 

The  various  remarkable  changes  which  the  lines  of  hy- 
drogen, magnesium,  sodium,  calcium,  and  iron  suffer  in  the 
spectrum  of  the  umbra,  seem  to  show  that  in  the  cloud-like 
and  vaporous  substances  constituting  the  spot,  the  new  com- 
binations are  disposed  in  layers,  according  to  their  specific 
gravity.  Thus  hydrogen  gas  occupies  the  highest  stratum  ; 
aqueous  vapor,  magnesium,  and  sodium  follow  in  thinner 
layers  below ;  and  the  heavier  vapors  of  calcium,  titanium, 
and  iron  form  the  lowest  and  densest  stratum,  the  base  of 
the  spot. 

The  formation  of  a  spot  will  accordingly  immediately  fol- 
low an  eruption  of  hydrogen  :  the  spot  itself  is  a  dense, 
cloudy,  luminous  mass,  probably  of  a  semi-fluid  consistency, 
composed  of  many  constituents  —  according  to  Zollner,  a 
kind  of  scoria  —  which  sinks  by  its  gravity  a  certain  depth 
into  the  photosphere,  or  outer  portion  of  the  sun,  and  par- 
tially intercepts  the  light  from  the  lower  stratum  of  the  pho- 
tosphere, therefore  presenting  to  us  the  appearance  of  a  dark 
mass  projected  upon  the  disk  of  the  sun,  in  the  same  way  as 
the  exceedingly  intense  light  of  the  oxyhydrogen  lime-light 
appears  black  when  seen  against  the  sun. 

The  enormous  dimensions  of  these  dense  masses  of  vapor, 
which  extend  sometimes  in  all  directions,  account  for  the 
length  of  time  the  spots  continue  visible,  not  unfrequently 

ii 


106  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLEN. 

remaining  during  several  rotations  of  the  sun.  Their  disap- 
pearance is  to  be  explained  partly  by  the  substance  of  the 
photosphere  flowing  into  the  cavity  of  the  spot,  partly  by 
the  complete  subsidence  of  the  vapors  into  the  nucleus  of 
the  sun,  where,  in  consequence  of  the  enormous  heat,  the 
compound  substances  which  may  exist  in  them  are  broken 
up  into  their  original  elements. 

These  conjectures  are  by  no  means  intended  to  afford  a 
complete  explanation  of  all  the  phenomena  of  a  solar  spot. 
Though  it  certainly  is  of  the  highest  interest  for  us  to  acquire 
a  knowledge  of  the  physical  nature  of  that  heavenly  body 
whence  we  derive  light,  heat,  motion,  and  life,  we  must  yet 
be  cautious  of  receiving  for  truth  what  is  only  the  result  of 
speculation,  especially  as  the  theories  on  this  subject  rest  on 
isolated  observations  which  are  too  unconnected  to  point  to 
any  certain  conclusion.  The  suggestions  here  thrown  out 
are  only  intended,  therefore,  to  throw  some  light  upon  the 
results  hitherto  obtained  by  the  spectrum  observations  of 
Secchi,  Huggins,  Lockyer,  and  Young,  and  by  affording  an 
unconstrained  interpretation  of  them  to  bring  them  into  har- 
mony with  the  phenomena  observed  during  the  total  solar 
eclipses  of  1868,  1869,  and  1871. 

TOTAL  SOLAR  ECLIPSES. 

The  reason  why  our  knowledge  concerning  the  nature  of 
the  sun  is  still  so  imperfect,  is  that  the  remarkable  phenom- 
ena occurring  on  the  sun's  limb  are  so  completely  over- 
powered by  the  blinding  light  of  the  solar  nucleus  or  photo- 
sphere, that  they  remain  invisible  even  in  the  most  powerful 
telescopes.  It  is  not  sufficient  to  get  rid  of  the  sun's  rays  by 
the  interposition  of  an  opaque  screen,  because  the  diffused 
light  of  the  sky  cannot  be  eliminated  by  this  means,  and  this 
light,  even,  is  so  intense  as  to  conceal  the  faint  light  of  the 
sun's  appendages.'  It  is  quite  otherwise,  however,  during  a 
total  eclipse  of  the  sun  ;  then  the  moon  covers  the  whole  of 

12 


SCHHLLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  107 

the  sun's  disk,  and  includes  a  large  tract  of  the  earth's  sur- 
face in  the  cone  of  its  shadow,  revealing  to  the  observer, 
who  is  no  longer  hindered  by  the  light  of  day,  a  display  of 
phenomena  round  the  sun  which  can  be  seen  in  no  other 
way,  and  the  study  of  which  is  peculiarly  fitted  to  throw 
light  on  the  nature  and  physical  constitution  of  the  sun. 

We  will  not  suffer  ourselves  to  be  detained  by  a  descrip- 
tion of  those  changes  that  pass  over  the  landscape  as  the 
darkness  advances,  nor  dwell  upon  the  deep  impression 
which  the  sudden  disappearance  of  the  last  rays  of  the  sun, 
and  the  equally  sudden  reappearance  of  the  light,  make 
both  upon  men  and  animals. 

The  diameter  of  the  cone  of  the  shadow  thrown  by  the 
moon  towards  the  earth,  amounts  at  the  spot  where  it 
touches  the  earth's  surface  on  the  equator  during  the  time  of 
totality  to  about  122  miles:  as,  however,  the  moon,  which 
throws  the  shadow,  only  completes  its  course  in  the  heavens 
round  the  earth  from  west  to  east  in  one  month,  and  the 
earth,  which  receives  the  shadow,  accomplishes  its  revolu- 
tion from  west  to  east  in  one  day,  it  follows  that  the  motion 
of  the  moon's  shadow  is  very  much  slower  than  that  of  the 
earth's  surface.  It  therefore  happens  that  the  earth  appears 
to  run  away  from  under  the  moon's  shadow,  or  that  the 
moon's  shadow  seems  to  run  over  the  earth  from  east  to  west. 
From  an  elevated  position  the  shadow  of  the  moon  is  seen 
to  approach  with  enormous  rapidity,  and  the  sensation  as 
though  a  material  substance,  such  as  a  terrific  cloud  of 
smoke,  were  rushing  over  the  earth's  surface,  fills  the  unini- 
tiated spectator  with  fear  and  dread.  A  few  minutes  before 
the  commencement  of  the  totality,  the  brightest  stars  become 
visible,  and  the  sharply  defined  black  edge  of  the  moon  ap- 
pears surrounded  on  all  sides  by  a  very  narrow  but  very 
brilliant  ring  of  light,  of  silver  whiteness,  which  is  called 
the  corona.  From  the  corona  faint  rays  of  light,  irregular 
in  length  and  breadth,  stream  out  in  all  directions,  surround- 

13 


IOS  RECREATIONS  IN  POPULAR  SCIENCE.     [SCHKLLEK. 

ing  the  moon's  disk  like  a  glory,  whence  this  crown  of  rays 
is  usually  designated  the  glory  or  halo. 

When  the  total  darkness  has  commenced,  the  prominences 
make  their  appearance,  which  are  cloud-like  masses  of  a 
rose  or  pale  coral  color,  disposed  either  singly  or  in  groups, 
at  various  places  on  the  moon's  limb. 

They  pierce  the  corona  in  the  most  wonderful  forms, 
sometimes  as  single  outgrowths  of  enormous  height,  some- 
times as  low  projections  spreading  far  along  the  moon's 
limb.  The  prominences  are  generally  first  seen  on  the 
eastern  (left)  side  of  the  sun,  where  at  the  commencement 
of  the  totality  the  moon  only  grazes  the  sun's  edge,  and  the 
space  immediately  surrounding  the  sun  is  yet  uncovered;  in 
proportion  as  the  moon  advances  to  the  east,  the  space  im- 
mediately surrounding  the  western  parts  of  the  sun  becomes 
free,  and  the  prominences  are  then  seen  also  on  that  side  in 
greater  number,  and  developed  with  much  greater  dis- 
tinctness. 

There  remains  now  no  longer  any  doubt  that  these  re- 
markable phenomena  belong  to  the  sun,  and  are  great  accu- 
mulations of  the  luminous  gaseous  material  by  which  the 
solar  body  is  wholly  surrounded  ;  it  cannot  therefore  greatly 
astonish  us  that  their  forms  have  been  seen  to  change  even 
during  the  short  duration  of  the  totality ;  that  which  calls 
much  more  for  wonder  is  the  enormous  height  to  which 
these  pillars  of  gas  extend  beyond  the  limb  of  the  sun,  a 
height  which  in  some  instances  exceeds  ninety  thousand 
miles. 


THE  TOTAL  SOLAR  ECLIPSE  OF  THE  7™  OF  AUGUST,  1869. 

This  eclipse  was  invisible  in  Europe  ;  the  zone  of  totality 
stretched  from  Alaska,  where  the  eclipse  began  at  noon, 
over  British  America  and  the  south-west  corner  of  Minne- 
sota, then  crossed  the  Mississippi  near  Burlington  (Iowa), 
and  passed  through  Illinois,  Western  Virginia,  and  North 


SCHELLEN.]      SPECTRUM  ANALYSIS  DISCOVERIES.  109 

Carolina,  reaching  the  Atlantic  Ocean  in  the  neighborhood 
of  Beaufort. 

The  event  excited  the  most  lively  interest  among  astrono- 
mers and  photographers  throughout  the  whole  of  North 
America,  and  occasioned  the  equipment  of  a  number  of  sci- 
entific expeditions,  which  were  also  supplemented  by  the 
valuable  labors  of  many  private  individuals.  The  observers 
were  in  almost  every  instance  favored  with  the  finest  weather, 
and  their  efforts  were  rewarded  by  a  large  collection  of  pho- 
tographic pictures,  and  many  valuable  spectroscopic  and 
other  observations.  That  portion  of  the  zone  of  totality 
which  traversed  the  inhabited  parts  of  the  United  States  was 
studied  everywhere  with  telescopes,  spectroscopes,  and  other 
instruments  of  observation,  so  that  the  whole  of  this  tract  of 
country  became  one  vast  observatory.  Although  the  dura- 
tion of  totality  was  less  than  in  the  eclipse  observed  in  In- 
dia (1868),  yet  the  phenomenon  was  attended  on  the  whole 
with  many  more  favorable  circumstances ;  the  heat  was  less 
intense,  the  places  suitable  for  observation  were  much  more 
conveniently  situated,  and  the  sun's  altitude  was  not  so  great 
as  in  the  eclipse  of  1868.  The  most  important  points  of 
investigation  had  reference  to  the  scrutiny  of  the  promi- 
nences by  means  of  photography  and  the  spectroscope,  the 
examination  of  the  nature  of  the  corona,  and  the  search  for 
planets  between  Mercury  and  the  Sun. 

The  most  complete  expeditions  were  those  sent  out  from 
Washington,  one  from  the  Nautical  Almanac  Office,  the  as- 
tronomical department  being  under  the  charge  of  Professor 
Coffin,  while  the  photographic  arrangements  were  conducted 
by  Professor  Henry  Morton,  of  Philadelphia :  another  expe- 
dition was  despatched  from  the  United  States  Naval  Obser- 
vatory, under  the  superintendence  of  Commodore  B.  F. 
Sands. 

The  first  expedition,  under  the  guidance  of  Professor 
Morton,  selected  stations  in  the  State  of  Iowa,  as  follows :  — 

i.  Burlington,  where  the  observers  were  Professor  Mayer, 

15 


110  RECREATIONS    IN  POPULAR    SCIENCE.    [SCHELLEN. 

and  Messrs.  Kendall,  Willard,  Phillipps,  and  Mahoney,  to- 
gether with  Dr.  C.  A.  Young,  Professor  of  Dartmouth  College 
(Hanover),  well  known  as  an  experienced  spectroscopist, 
and  Dr.  B.  A.  Gould,  to  whose  charge  the  photographic 
department  was  committed ; 

2.  Ottumwa,  where  Professor  Hiines,  and  Messrs.  Zent- 
mayer,  Moelling,  Brown,  and  Baker,  were  stationed  ; 

3.  Mount  Pleasant,  occupied  by  Professor  Morton,  and 
Messrs.  Wilson,  Clifford,  Cremer,  Ranger,  and  Carbutt,  as 
well  as  by  some  other  Professors,  including  Pickering,  who 
were  desirous  of  making  astronomical  observations  on  the 
physical  phenomena  of  the  eclipse. 

Stations  selected  by  the  second  expedition :  — 

1.  Des  Moines  (Iowa),  where  Professor  Newcomb  under- 
took the  observation  of  the  corona  and  the  search  for   in- 
termercurial    planets,  Professor  Harkness  the  spectroscopic 
investigations,    and   Professor   Eastman   the    meteorological 
department.     Several  other  gentlemen  skilled  in  solar  pho- 
tography associated  themselves  with  these  observers. 

2.  Bristol  (Tennessee),  where  Bard  well,  who  undertook 
the  observation  of   the   corona,  and   other  observers  were 
stationed. 

Besides  these  most  important  expeditions,  furnished  with 
the  most  admirable  and  complete  means  of  observation,  sev- 
eral scientific  men  were  engaged  at  various  points  in  the 
zone  of  totality,  either  in  observing  the  astronomical  details 
of  the  eclipse,  or  in  investigating  the  prominences,  the  coro- 
na, and  their  spectra.  Among  these  may  be  mentioned  Dr. 
Edward  Curtis,  who  at  Des  Moines  obtained  no  fewer  than 
one  hundred  and  nineteen  pictures  of  the  different  phases  of 
the  eclipse ;  W.  S.  Oilman,  by  whom  some  most  valuable 
observations  were  instituted  at  St.  Paul  Junction  (Iowa) 
upon  the  connection  between  the  solar  spots,  the  faculaB,  and 
the  prominences ;  J.  A.  Whipple,  who  with  Professor  Win- 
lock  and  several  assistants  procured  at  Shelbyville  (Ken- 
tucky) eighty  photographic  pictures,  six  of  which  were  taken 

16 


SCHKLLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  Ill 

during  the  totality,  one  of  them  exhibiting  a  complete  and 
magnificent  corona ;  as  well  as  Professor  G.  W.  Hough, 
Director  of  the  Dudley  Observatory,  who  in  company  with 
nine  fellow-observers  recorded  all  the  details  of  the  eclipse 
at  Mattoon  (Illinois). 

Out  of  the  mass  of  materials  afforded  by  the  observations 
of  this  eclipse,  it  will  only  come  within  our  province  to 
communicate  those  results  which  have  reference  to  the  phys- 
ical constitution  of  the  sun,  and  were  obtained  partly  by 
photographic  delineation,  and  partly  by  the  help  of  the 
spectroscope.  The  course  of  the  eclipse  and  the  photo- 
graphic work  carried  on  at  Mount  Pleasant,  where,  the  total- 
ity lasted  two  minutes,  forty-eight  seconds,  is  described  by 
Wilson  nearly  as  follows  :  — 

"  For  some  days  prior  to  the  eclipse,  the  sky  was  over- 
cast, and  threatened  rain  ;  but  the  7th  of  August  was  bright, 
without  a  cloud,  such  a  day  as  had  not  occurred  for  months, 
and  the  sun  shone  with  remarkable  clearness  and  warmth. 
The  moment  of  first  contact  arrived ;  the  first  plate  was 
already  placed  in  the  tube ;  Professor  Watson  signalled  to 
us  the  moment  for  exposure  by  a  motion  of  the  hand  ;  the 
instantaneous  shutter  was  opened  and  closed,  and  the  first 
picture,  was  taken.  We  thus  commenced  a  series  of  pictures 
taken  at  intervals  of  five  or  ten  minutes  till  the  commence- 
ment of  totality,  after  which  the  series  was  continued  on  the 
re-appearance  of  the  sun  till  the  termination  of  the  eclipse. 
Darkness  came  on  with  the  totality,  but  not  the  darkness  of 
night ;  still  it  rendered  reading  impossible.  The  amount  of 
light  upon  the  landscape  was  scarcely  equal  to  that  of  bright 
moonlight,  yet  it  was  sufficient  for  us  to  pursue  our  work. 
An  instant  before  the  commencement  of  totality,  the  thin 
crescent  of  the  sun  was  still  quite  dazzling ;  then  the  light 
went  out  as  from  an  expiring  candle. 

"  There,  between  heaven  and  earth,  hung  face  to  face  the 
two  great  luminaries,  sun  and  moon,  a  large  black  round 
spot  encircled  by  a  brilliant  ring  of  deep  gold-colored  light, 

'7 


112  RECREATIONS   IN   POPULAR  SCIENCE.  [SCHELLEM. 

interrupted  here  and  there  by  the  brighter  spots  of  the 
flesh-colored  prominences  of  irregular  size  and  form,  and 
surrounded  by  the  magnificent  corona,  which  shot  out  rays 
in  every  direction,  faintest  where  the  prominences  were 
most  conspicuous,  but  enveloping  the  whole  with  a  glory 
which  was  marvellously  beautiful,  as  if  the  Creator  were 
about  to  show  His  omnipotence  in  this  wonder.  The  phe- 
nomenon resembled  a  gigantic  image  from  a  magic  lantern, 
received  upon  the  heavens  as  a  screen.  Four  plates  were 
exposed,  when  suddenly  the  full  significance  of  those  words 
was  realized,  '  Let  there  be  light,  and  there  was  light,'  for  a 
mighty  flood  of  brilliant  light  gushed  forth,  like  the  rushing, 
foaming  waters  of  Niagara.  The  sun  came  forth  like  a 
conqueror  from  a  battle  with  the  Titans,  and  was  greeted 
with  acclamations  by  the  assembled  spectators." 

A  picture  of  this  magnificent  spectacle  is  given  in  Fig.  VII., 
showing  the  prominences  and  corona  after  a  photograph  by 
Professor  Eastman,  which  was  taken  at  the  commencement 
of  the  totality.  The  instant  the  totality  began,  the  corona 
made  its  appearance  as  a  light  of  silvery  whiteness,  with  an 
exceedingly  tender  flush  of  a  greenish-violet  hue  at  the  ex- 
treme edges,  and  not  the  slightest  change  was  perceptible 
during  the  totality  in  the  color,  the  outline,  or  the  position 
of  the  rays  —  an  observation  confirmed  by  Professor  Hough 
at  Mattoon  (Illinois),  by  Gill,  and  by  several  others. 

The  corona  appeared  to  consist  of  two  principal  portions  : 
the  inner  one,  next  to  the  sun,  was  nearly  annular,  reaching 
an  elevation  of  about  i',  and  in  color  of  a  pure  silvery  white- 
ness ;  the  outer  portion  consisted  of  rays,  some  of  which 
grouped  themselves  into  five  star-like  points,  while  the  others 
assumed  the  appearance  of  radiations,  and  were  the  most 
sharply  defined  ;  the  corona  was  scarcely  visible  between  the 
prominences  a  and  b.  The  star-like  rays  attained  a  height 
equal  to  half  the  diameter  of  the  sun. 

The  observations  made  by  several  astronomers  in  India 
during  the  eclipse  of  1868,  and  those  made  by  many  others 

18 


SCHELLEN.]      SPECTRUM  ANALYSIS  DISCOVERIES.  113 

in  America  during  that  of  1869,  and  still  later  those  by 
Lockyer,  Janssen,  and  others  in  and  near  the  island  of  Cey- 
lon, in  December,  1871,  fully  justify  the  conclusion  that,  in- 
dependently of  the  cosmical  materials  which  must  exist  in 
the  neighborhood  of  the  sun,  there  exists  around  this  body 

FIG.  VII. 


The  Corona  of  the  Eclipse  of  7th  August.  1869,  at  Des  Moines. 

an  atmosphere  very  extensive  and  excessively  rare,  with 
hydrogen  for  its  basis.  This  atmosphere,  which  undoubt- 
edly forms  the  outside  gaseous  envelope  of  the  sun,  is  fed  by 
the  material  of  the  protuberances  projected  with  such  vio- 
lence from  the  bowels  of  the  photosphere,  but  is  distin- 

19 


114  RECREATIONS    IN   POPULAR   SCIENCE.    [SCHELLEN. 

guished  from  the  chromosphere  and  the  protuberances  by 
a*  density  enormously  less,  a  lower  temperature,  and  perhaps 
by  the  presence  of  certain  different  gases. 

The  prominences  are  masses  of  luminous  gas,  principally 
luminous  hydrogen  gas ;  they  envelop  the  entire  surface  of 
the  solar  body,  sometimes  in  a  low  stratum  extending  over 
exceedingly  large  tracts  of  the  sun's  surface,  sometimes  in 
accumulated  masses  rising  at  certain  localities  to  a  height 
of  more  than  80,000  miles. 

They  are,  as  respects  their  first  formation,  phenomena  of 
eruption.  The  velocity  with  which  the  gaseous  matter  of 
the  prominences  must  pass  the  photosphere  must  be,  in  many 
cases  at  least,  two  hundred  miles  per  second,  and  its  initial 
velocity  probably  not  less  than  three  hundred  miles  per  sec- 
ond. Dense  gaseous  matter  flung  out  with  the  hydrogen 
would  probably  retain  a  velocity  of,  say  two  hundred  and 
forty  miles  per  second,  and  reach  a  height  exceeding  that  in- 
dicated by  the  greatest  extension  of  the  radiations  observed 
last  December. 

The  body  of  the  sun,  or  its  light-giving  envelope,  the  pho- 
tosphere, is  completely  surrounded  by  a  gaseous  envelope,  in 
which  hydrogen  constitutes  the  chief  element,  and  which  is 
called  the  chromosphere.  Its  mean  thickness  is  between 
five  thousand  and  seven  thousand  miles. 

The  prominences  are  local  accumulations  of  the  chromo- 
sphere, and  therefore  pre-eminently  of  hydrogen  gas,  which 
appear  to  break  forth  from  time  to  time  from  the  interior  of 
the  sun  in  the  form  of  monster  eruptions,  forcing  their  way 
through  the  photosphere  and  chromosphere.  As  this  gas, 
on  effecting  a  passage,  rises  with  great  rapidity,  it  becomes 
quickly  rarefied  in  a  direction  away  from  the  sun's  limb. 

From  the  experiments  undertaken  by  Lockyer,  Frankland, 
Wiillner,  and  Secchi,  it  appears  that  even  in  the  lowest  stra- 
tum of  this  gaseous  envelope,  the  pressure  is  smaller  than 
that  of  our  atmosphere,  therefore  that  the  gas  of  the  chro- 
mosphere is  in  a  state  of  greater  attenuation. 

20 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  115 

Under  the  chromosphere  lies  the  luminous  cloud-like 
vaporous  or  nebulous  photosphere,  which  contains  all  the 
substances,  the  spectrum  lines  of  which  appear  as  absorption 
lines  in  the  solar  spectrum.  These  substances  —  among 
which  iron,  magnesium,  and  sodium  are  especially  prom- 
inent—  often  burst  forth  in  a  state  of  incandescence,  and  are 
carried  up  to  a  certain  distance  into  the  chromosphere,  and 
into  the  basis  of  the  prominences,  though  not  in  general  to 
any  considerable  elevation. 

It  is  probable  that,  owing  to  a  continuous  decrease  in  its 
temperature  and  density,  the  chromosphere  stretches  out 
into  space  to  a  distance  far  beyond  our  power  of  recognition. 

MODES  OF  OBSERVING  THE  PROMINENCES  IN  SUNSHINE. 
FORM  OF  THE  PROMINENCES. 

As  early  as  1866,  Lockyer  attempted  to  observe  the  prom- 
inences in  full  sunshine  by  means  of  a  Herschel-Browning 
spectroscope  placed  in  combination  with  a  telescope.  The 
method  he  employed,  and  which  he  laid  before  the  Royal 
Society  in  a  special  communication,  depends  on  the  specific 
difference  between  the  light  of  the  prominences  and  that  of 
the  sun  itself. 

The  light  of  an  incandescent  solid  or  liquid  body  which 
passes  through  the  slit  of  a  spectroscope  will  be  spread  out 
by  the  prism  into  a  band  of  greater  or  less  length,  and  form 
a  continuous  spectrum. 

The  light  of  a  gaseous  or  vaporous  body  will  by  the  same 
means,  on  the  contrary,  be  decomposed  into  a  few  only, 
sometimes  even  into  a  very  few,  bright  lines. 

In  the  first  case,  the  greater  the  length  of  the  spectrum, 
the  less  will  be  its  intensity  in  comparison  with  that  of  the 
source  of  light ;  in  the  second  case,  especially  when  the 
spectrum  consists  only  of  a  couple  of  lines,  the  intensity 
of  each  line  is  little  less  than  half  that  of  the  light  itself. 

If,  therefore,  an  equal  amount  of  light  from  two  self 

21 


Il6  RECREATIONS    IN    POPULAR   SCIENCE.   [SCHELLEX. 

luminous  bodies,  one  of  which  is  solid  or  liquid,  and  the 
other  gaseous  or  vaporous,  enter  the  slit  of  the  spectroscope 
at  the  same  time,  the  bright  lines  of  the  latter  will  be  more 
brilliant  than  the  color  of  the  corresponding  portion  of  the 
continuous  spectrum. 

Now,  by  increasing  the  number  of  prisms,  the  continuous 
spectrum  may  become  so  elongated,  and  consequently  dimin- 
ished in  light,  that  the  once  brilliant  solar  spectrum  may  be 
reduced  to  the  verge  of  visibility,  while  the  same  amount  of 
dispersion  produces  on  a  spectrum  of  lines  from  glowing 
gas  only  an  increase  in  the  distance  between  the  lines,  and 
no  considerable  diminution  of  their  brilliancy. 

The  reason  why  the  prominences  round  the  sun's  limb 
cannot  be  seen  through  a  telescope  at  any  time  by  screening 
off  the  intense  light  of  the  sun,  is  owing  to  the  extreme  bril- 
liancy with  which  the  sun  illuminates  the  earth's  atmos- 
phere, the  particles  of  which  scatter  so  large  an  amount  of 
light  as  quite  to  overpower  the  fainter  light  of  the  prom- 
inences, and  prevent  them  making  any  sensible  impression 
on  the  eye. 

In  a  total  eclipse  of  the  sun,  the  light  of  this  atmosphere 
is  so  considerably  reduced  as  to  allow  the  larger  prominences 
beyond  the  limb  of  the  sun  to  be  observed  by  the  unassisted 
eye.  The  possibility  of  reducing  the  glare  of  sunlight  at 
any  other  time  without  extinguishing  the  light  of  the  prom- 
inences, rests  on  the  circumstance  already  mentioned,  that 
the  light  of  the  sun  consists  of  rays  of  every  color,  and 
therefore  produces  in  a  spectroscope  of  highly  dispersive 
power  a  long  and  faint  spectrum,  while  the  light  of  the 
prominences,  consisting  in  general  of  only  three  or  four 
kinds  of  rays,  remains  even  after  the  greatest  dispersive 
power  still  concentrated  into  the  same  number  of  lines 
(H«,H?,Hy,D3). 

It  was  on  these  principles,  first  announced  by  Lockyer, 
that  Janssen  succeeded,  the  day  after  the  eclipse  of  the  i8th 
of  August,  1868,  in  observing  the  spectrum  of  the  prom- 

22 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  1 17 

inences  in  sunshine.  That  the  method  he  employed  was  no 
other  than  that  suggested  by  Lockyer,  is  evident  from  his 
own  communication  to  the  French  Academy,  dated  Calcutta, 
the  3d  of  October,  1868,  in  which  he  expressed  himself  as  fol- 
lows :  "  The  principle  of  the  new  method  rests  upon  the 
difference  between  the  spectrum  peculiarities  of  the  light  of 
the  prominences  and  that  of  the  photosphere.  The  light  of 
the  photosphere,  which  is  derived  from  incandescent  solid  or 
liquid  particles,  is  incomparably  stronger  than  that  of  the 
prominences,  which  is  derived  from  gases.  On  this  account 
it  has  been  impossible  hitherto  to  see  the  prominences,  ex- 
cept during  a  total  solar  eclipse.  By  the  employment,  how- 
ever, of  spectrum  analysis,  the  circumstances  of  the  case 
may  be  reversed.  In  fact,  by  the  process  of  analyzation, 
the  light  of  the  sun  is  dispersed  over  the  whole  range  of 
the  spectrum,  and  its  intensity  becomes  considerably  les- 
sened. The  prominences,  on  the  contrary,  furnish  only  a 
few  detached  groups  of  rays  which  are  bright  enough  to 
bear  comparison  with  the  corresponding  rays  of  the  solar 
spectrum.  It  is  for  this  reason  that  the  lines  of  the  prom- 
inences may  be  seen  easily  in  the  same  field  of  the  spectro- 
scope with  the  solar  spectrum,  while  the  direct  images  of 
the  prominences  are  invisible  on  account  of  the  overpower- 
ing light  of  the  sun.  Another  circumstance  very  favorable 
to  the  new  method  of  observation  lies  in  the  fact  that  the 
bright  lines  of  the  prominences  correspond  with  the  dark 
lines  of  the  solar  spectrum  :  they  can,  therefore,  not  only  be 
more  easily  recognized  in  the  field  of  the  spectroscope 
along  the  edges  of  the  solar  spectrum,  but  also  detected  on 
the  solar  spectrum  itself,  and  their  traces  even  followed  on 
the  very  surface  of  the  sun." 

As  soon  as  Janssen  and  Lockyer  had  succeeded  by  this 
method  in  observing  the  spectrum  of  the  prominences  inde- 
pendently of  a  total  eclipse,  it  became  a  question  whether  it 
would  not  be  possible  not  merely  to  see  the  lines  of  the 

23 


IlS  RECREATIONS  IN  POPULAR  SCIENCE.     [SCHELLEX. 

prominences,  but   also  to   make  their  actual   forms  visible 
during  sunshine. 

It  was  on  the  principles  before  mentioned  that  Lockyet 
based  his  plan  of  observing  the  spectra  of  the  prominences 
in  full  sunlight  by  means  of  a  telespectroscope  (Fig.  VIII.). 


FIG.  VIII. 


LOCKYER'S   TELESPECTROSCOPE. 

For  this  purpose  the  slit  of  a  highly  dispersive  spectroscope, 
d  c  e  /z,  firmly  attached  by  the  rods  a  a  b  to  an  equatorially 
mounted  telescope  L  T  P,  driven  by  clockwork,  is  directed 
perpendicularly  on  to  the  edge  of  the  sun's  image  formed  in 
the  telescope.  By  moving  the  tube  e  of  the  spectroscope 

24 


SCHBLLKM.]      SPECTRUM  ANALYSIS  DISCOVERIES.  119 

from  end  to  end  of  the  spectrum,  and  setting  the  focus  each 
time,  the  bright  lines  of  the  prominences  may  be  seen  as 
prolongations  of  the  dark  lines  of  the  spectrum  of  the  sun's 
disk  on  a  background  of  the  exceedingly  faint  spectrum  of 
the  earth's  atmosphere.  In  the  picture,  S  is  the  finder,  g  a 
handle  for  moving  the  telescope  in  declination,  d  the  tube 
containing  the  slit,  h  a  small  telescope  for  reading  the  divis- 
ions on  the  micrometer  screw  head,  partly  concealed  by  the 
rod  a  a. 

The  telespectroscope  is  furnished  with  seven  prisms,  and 
it  confirmed,  after  a  few  trials,  the  correctness  of  this  view, 
and  he  was  the  first  to  succeed,  without  additional  mechan- 
ical help  or  the  use  of  colored  glasses,  in  observing  the 
prominences  at  any  time  when  the  sun  was  visible,  and 
tracing  their  complete  outline. 

By  the  same  means  Zollner  saw  the  prominences  for  the 
first  time  on  the  ist  of  July,  1869.  He  has  published  the 
results  of  his  observations,  and  accompanied  them  by  a 
series  of  highly  interesting  drawings  of  some  of  the  larger 
prominences,  in  which  their  origin,  development,  and  subse- 
quent disappearance  are  very  clearly  exhibited. 

When  the  spectrum  of  the  earth's  atmosphere  has  disap- 
peared in  consequence  of  the  powerful  dispersion  of  the 
light,  and  the  portion  of  the  prominence  then  in  the  field  of 
view  alone  is  visible  through  the  widely  opened  slit,  the  tel- 
escope or  slit  is  moved  slowly  forward,  and  luminous  images 
of  the  most  wonderful  forms  flit  before  the  eye,  being  just  as 
easily  observed  as  during  a  total  solar  eclipse.  In  describing 
some  of  these  shadow  forms,  Lockyer  writes,  u  Here  one  is 
reminded,  by  the  fleecy,  infinitely  delicate  cloud-films,  of  an 
English  hedgerow  with  luxuriant  elms ;  here  of  a  densely 
intertwined  tropical  forest,  the  intimately  interwoven  branches 
threading  in  all  directions,  the  prominences  generally  ex- 
panding as  they  mount  upward,  and  changing  slowly, 
indeed  almost  imperceptibly.  ...  As  a  rule,  the  attach- 
ment to  the  chromosphere  is  narrow,  and  is  not  often. 

25 


120  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHKLLEN. 

single ;  higher  up,  the  stems,  so  to  speak,  intertwine,  and 
the  prominence  expands  and  soars  upward  until  it  is  lost 
in  delicate  filaments,  which  are  carried  away  in  floating 
masses." 

The  various  forms  of  the  prominences  may  be  classified 
generally  into  two  characteristic  groups,  very  aptly  desig- 
nated by  Zollner  as  vaporous  or  cloud-like  forms,  and  erup- 
tive forms. 

Slight  changes  in  the  form  of  the  prominences  may  be 
watched  almost  without  intermission  with  an  open  slit ; 
great  changes,  as  a  rule,  take  place  only  very  slowly,  or 
quite  imperceptibly.  In  .some  cases,  however,  the  change 
in  the  form  of  a  prominence  is  so  extraordinary,  and  occurs 
with  such  rapidity,  that  it  can  only  be  ascribed  to  extremely 
violent  agitation  in  the  upper  portions  of  the  solar  atmos- 
phere, compared  with  which  the  cyclonic  storms  occasion- 
ally agitating  the  earth's  atmosphere  sink  into  insignificance. 
The  observation  of  such  a  solar  storm  has  been  thus  de- 
scribed by  Lockyer : — 

"On  the  I4th  of  March,  1869,  about  9  h.  45m.,  with  a 
slit  tangential  to  the  sun's  limb  instead  of  radial,  which  was 
its  usual  position^  I  observed  a  fine  dense  prominence  near 
the  sun's  equator,  on  the  eastern  limb,  in  which  intense  ac- 
tion was  evidently  taking  place.  At  10  h.  50  m.,  when  the 
action  was  slackening,  I  opened  the  slit ;  I  saw  at  once  that 
the  dense  appearance  had  all  disappeared,  and  cloud-like 
filaments  had  taken  its  place.  At  1 1  h.  5  m.,  it  was  about 
twenty-seven  thousand  miles  high,  and  the  portion  on  the 
left  resembled  a  straight  column  in  a  slightly  leaning  posi- 
tion. I  left  the  Observatory  for  a  few  minutes,  and  on  re- 
turning, at  ii  h.  15  m.,  I  was  astonished  to  find  that  part  of 
the  straight  prominence  had  entirely  disappeared ;  not  even 
the  slightest  rack  appeared  in  its  place :  whether  it  was  en- 
tirely dissipated,  or  whether  parts  of  it  had  been  wafted 
towards  the  other  part,  I  do  not  know,  although  I  think  the 
latter  explanation  the  more  probable  one,  as  the  other  part 

26 


SCHELLKN.]    SPECTRUM   ANALYSIS   DISCOVERIES. 


121 


had    increased.     Fig.  IX.  shows  the   prominence  as  it  ap- 
peared at  the  last  observation. 


FIG.  IX. 


STORM  OBSERVED  BY  LOCKYER  ON  THE  14th  MARCH,  1859. 

Professor  C.  A.  Young,  of  Dartmouth  College,  Hanover, 
N.  II.,  has  devoted  himself  especially  to  the  observation  of 
the  forms  and  variability  of  the  prominences.  In  the  col- 
ored plate  accompanying  this  paper  are  represented  some 
of  the  forms  and  the  color  of  the  prominences.  The  one 
here  shown  is  of  the  vaporous  or  cloud-like  form,  and  was 
observed  by  Professor  Young,  October  7,  1869. 

The  change  in  the  form  between  the  first  and  second  ob- 
servation will  serve  to  illustrate  the  motions  of  the  hydrogen 
flames,  and  will  give  the  reader  some  slight  idea  of  the  dis- 
turbances which  are  constantly  occurring  on  the  surface  of 
the  sun.  By  means  of  the  accompanying  scale  their  height 

27 


122  RECREATIONS   IN   POPULAR   SCIENCE.   [SCHELLKM 

can  be  easily  ascertained.  Professor  Young  has  since  ob 
served  others  of  a  much  greater  height  and  magnitude. 

As  the  meteorologist  registers  many  times  in  a  day  the 
conditions  of  our  atmosphere,  in  the  hope  that  a  comparison 
of  the  observations  may  lead  to  a  discovery  of  the  law  gov- 
erning these  changes,  so  has  Respighi,  Director  of  the  Uni- 
versity Observatory  at  the  Campidoglio  at  Rome,  made  it 
his  daily  task  since  October,  1869,  to  observe  the  entire  limb 
of  the  sun,  when  the  weather  was  favorable,  including  the 
chromosphere  and  prominences,  and  to  mark  upon  a  straight 
line  representing  the  circumference  of  the  sun  the  position, 
height,  and  form  of  the  prominences  for  each  day.  By  col- 
lating these  lines,  or  circumferences  of  the  sun  one  bdow  the 
other,  and  crossing  them  with  lines  indicating  the  principal 
positions,  a  comprehensive  picture  is  afforded  of  the  distri- 
bution of  the  prominences  round  the  sun's  limb,  which 
shows  at  a  glance  those  regions  in  which  the  prominences 
abound,  and  those  in  which  they  are  least  frequently  to  be 
met  with. 

By  a  comparison  of  the  maps  already  constructed,  Res- 
pighi has  arrived  at  the  following  results  :  — 

1.  In  the  polar  regions  prominences   occur  only  excep- 
tionally.    The  district  from  which  they  are  absent  lies  be- 
tween north   and  north-east  on  the  one  side,  and  south  and 
south-west  on  the  other  ;  the  portion  which  is  almost  entirely 
without  prominences  has  a  semi-diameter  of  22^°. 

2.  The  district  where  the  prominences   most  frequently 
occur  lies  between  north  and  north-west,  at  about  45°  north 
latitude,  in  a  region  where  solar  spots  are  rarely  seen. 

3.  The  prominences  are,  therefore,  phenomena  quite  dis- 
tinct from  the  spots ;  they  are  probably  more  intimately  con- 
nected with  the  formation  of  faculcc. 

4.  The  various  forms  of  the  prominences  show  that  they 
are  not  of  the  nature  of  clouds,  which  float  in  an  atmosphere 
in  which  they  are  produced   by  local  condensations ;   they 
are  much   more   like  eruptions  out  of   the  chromosphere, 

28 


Solar  Prominence  observed  ~byYoun 
1869-Oor  7 


Time  Oct. 7.  2* 


,*/*>  INS  L,rn  CO  IOS.SV** 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  123 

which  often  spread  out  of  the  higher  regions,  and  take  the 
form  of  bouquets  of  flowers,  some  being  bent  over  on  one 
side,  and  some  on  the  other,  and  which  fall  again  on  to  the 
surface  of  the  chromosphere  as  rapidly  as  they  rose  from  it. 

5.  It  appears  that  eruptions  of  hydrogen  take  place  from 
the  interior  of  the  sun  ;  their  form  and  the  extreme  rapidity 
of  their  motion  necessitates  the  hypothesis  of  a  repulsive 
power,  at  work  either  at  the  surface  or  in  the  mass  of  the 
sun,  which  Respighi  attributes  to  electricity,  but  Faye  simply 
to  the  action  of  the  intense  heat  of  the  photosphere. 

On  the  28th  of  September,  1870,  Professor  Young  suc- 
ceeded for  the  first  time  in  photographing  the  prominences 
on  the  sun's  limb  in  bright  sunshine.  This  he  effected  by 
bringing  the  blue  hydrogen  line  H  y  near  G  into  the  middle 
of  the  field  of  the  spectroscope,  and  placing  a  small  photo- 
graphic camera  in  connection  with  the  eye-piece  of  the  tel- 
escope. As  the  chemicals  employed  were  those  ordinarily 
used  in  taking  portraits,  the  requisite  time  of  exposure  was 
three  and  one  half  minutes,  during  which  time  the  image  of 
the  prominence  suffered  a  slight  displacement  on  the  pre- 
pared plate,  owing  to  a  want  of  accuracy  in  the  perfect  ad- 
justment of  the  polar  axis.  Still,  however,  the  various 
forms  of  the  prominences  could  be  clearly  discerned  in  the 
photograph,  which  was  half  an  inch  in  diameter,  so  that  the 
possibility  of  photographing  the  prominences  has  been 
proved  by  Young's  experiment. 

MEASUREMENT  OF  THE   DIRECTION  AND   SPEED   OF  THE 
GAS-STREAMS  IN  THE  SUN. 

One  of  the  most  glorious  triumphs  of  spectrum  analysis  — 
surpassing,  perhaps,  in  splendor  all  its  other  wonderful 
achievements  —  is  the  discovery  that,  by  means  of  accurate 
measurements,  undertaken  with  the  best  instruments,  of  the 
position,  or  rather  of  the  small  displacement  in  the  position 
of  the  spectrum  lines  of  a  star  or  other  source  of  light,  —  a 

29 


124  RECREATIONS  IN  POPULAR  SCIENCE.     [SCHBLLEN. 

prominence-  for  instance,  —  it  is  possible  to  ascertain  whethei 
this  luminous  body  be  approaching  us  or  receding  from  us, 
and  at  what  speed  it  is  travelling. 

The  pitch  of  a  musical  tone  depends,  as  is  well  known, 
upon  the  number  of  impulses  which  the  ear  receives  from 
the  air  in  a  given  time  (p.  74).  Now,  as  a  tone  rises  in  pitch 
the  greater  the  number  of  air  vibrations  which  strike  the 
tympanum  in  a  second,  so  must  a  sound  ascend  in  tone  if  we 
rapidly  approach  it,  and  fall  in  pitch  if  we  recede  from  it. 
The  truth  of  this  supposition  may  be  fully  proved  by  the 
whistle  of  a  railway  engine  in  rapid  motion.  To  an  ob- 
server standing  still,  the  pitch  of  the  tone  rises  on  the  rapid 
approach  of  the  locomotive,  although  the  same  note  is 
sounded,  and  falls  again  as  the  engine  travels  away. 

As  the  various  tones  of  sound  depend  on  the  rapidity  of 
the  air  vibrations,  so  the  varieties  of  color  are  regulated  by 
the  number  of  ether  vibrations  (p.  77)-  If>  therefore,  a  lu- 
minous object  —  as,  for  instance,  the  glowing  hydrogen  of  a 
prominence  —  be  receding  rapidly  from  us,  fewer  waves  of 
ether  will  strike  the  optic  nerve  in  a  second  than  if  it  were 
stationary.  If  the  difference  in  the  number  of  ether  waves 
be  sufficiently  great  to  be  perceived  by  the  eye,  then  each 
color  of  the  glowing  gas  must  sink  in  the  scale  of  the  spec- 
trum,—  that  is  to  say,  incline  more  towards  the  red.  The 
individual  colored  rays  will  not  then,  in  the  prismatic  de- 
composition of  the  light,  occur  in  the  same  place  of  the 
spectrum  in  which  they  would  have  appeared  had  the  light 
been  stationary ;  they  will  all  be  displaced  somewhat 
towards  the  red. 

The  converse  takes  place  when  the  luminous  body  is  rap- 
idly approaching  us :  the  number  of  ether  vibrations  re- 
ceived by  the  eye  is  then  increased  beyond  what  it  would  be 
if  the  source  of  light  were  stationary  ;  in  the  prismatic  anal- 
ysis of  the  light,  the  colored  rays  will  be  found  likewise  to 
have  changed  their  place  in  the  scale  of  the  spectrum,  and 
taken  a  position  in  accordance  with  their  increased  re- 

30 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES. 


I25 


frangibility,  suffering  a  general  displacement  towards  the 
violet. 

When  it  is  remembered  that  the  number  of  ether  waves  in 
red  light  is  at  least  480  billion,  and  in  violet  800  billion  in  a 
second,  and  that  moreover  the  wave  length  of  the  greenish- 
blue  light  (H  £),  situated  at  the  spot  marked  F  in  the  solar 
spectrum,  is  only  485  millionth  of  a  millimetre,  and  that 
instruments  of  sufficient  delicacy  to  measure  these  minute 
quantities  are  required  for  this  purpose,  there  will  be  little 
danger  of  under-estimating  the  extreme  difficulty  connected 
with  observations  of  this  displacement  in  the  colors  of  the 
spectrum.  Indeed,  these  observations  would  scarcely  be 
possible,  were  it  not  that  in  the  dark  lines  crossing  the  spec- 
tra of  the  sun  and  fixed  stars,  the  places  of  some  of  which 
may  be  accurately  ascertained,  we  have  fixed  positions  in 
the  spectrum,  the  degree  of  refrangibility  or  wave-length  of 
which  may  be  determined  beforehand,  both  for  the  sun  and 
terrestrial  substances,  and  also  for  the  stars  or  other  sources 
of  light  supposed  to  be  at  rest. 

Fig.  X.,  which  is  from  a  drawing  by  Lockyer,  shows 
clearly  what  remarkable  changes  take  place  in  the  dark  line 

FIG.  X. 


Displacement  of  the  F-line  ;  Velocity  of  the  Gas-streams  in  the  Sun. 

F  when  the  spectroscope  is  directed  to  a  solar  spot  in  the 
middle  of  the  sun.  The  F-line,  which,  as  a  rule,  is  sharply 
defined  at  the  edges,,  appears  in  some  places  not  merely  as  a 

31 


126  RECREATIONS   IN   POPULAR   SCIENCE.   [SCHELLBN. 

bright  line,  but  as  a  bright  and  dark  line  twisted  together,  in 
which  parts  it  suffers  the  greatest  displacement  towards  the 
red.  When  this  occurs,  there  is  frequently  also  a  bright  line 
to  be  seen  on  the  violet  side.  In  small  solar  spots,  this  line 
sometimes  breaks  off  suddenly,  or  spreads  out  immediately 
before  its  termination  in  a  globular  form ;  over  the  bright 
faculae  of  a  spot  (the  bridges)  the  line  is  often  altogether 
wanting,  or  else  it  is  reversed,  and  appears  as  a  bright  line. 

The  same  phenomena  are  exhibited  also  by  the  red  C-line 
(H«),  though  as  the  greenish-blue  F-line  (H  @)  is  by  an 
equal  increase  of  pressure  much  more  sensitive  with  regard 
to  expansion  than  the  red  line  is,  and  exhibits  with  greater 
distinctness  the  changes  that  have  been  already  described,  it 
is  better  adapted  to  observations  of  this  kind. 

All  these  expansions,  twistings,  and  displacements  of  the 
F-line  result  from  a  change  in  the  wave-length  of  the  green- 
ish-blue light  emitted  by  the  moving  masses  of  incandescent 
hydrogen  gas  in  the  sun.  The  middle  of  this  line,  when  it 
is  well  defined,  corresponds  to  a  wave-length  of  485  mil- 
lionth of  a  millimetre,  yet  it  is  possible,  by  means  of  Ang- 
strom's maps  of  the  solar  spectrum,  to  measure  a  displace- 
ment of  this  line  when  the  wave-length  has  only  changed  as 
much,  as  TUMnrVznjiy  of  a  millimetre,  and,  inversely,  it  is  also 
possible  to  read  off  at  once,  by  the  measured  displacement 
of  the  F-line,  the  corresponding  amount  which  the  wave- 
length of  the  greenish-blue  hydrogen  light  has  lengthened  or 
shortened,  to  ten  millionth  of  a  millimetre.  In  observing  a 
prominence,  if  the  F-line  were  to  be  displaced  from  its  nor- 
mal place  in  the  solar  spectrum  iwrfuwu  °f  a  millimetre, 
the  light  would  be  approaching  the  eye  of  the  observer,  and 
an  eruption  of  gas  be  ascending  at  the  spot  observed  in  the 
middle  of  the  sun,  and  would  be  approaching  the  earth  at 
the  rate  of  thirty-six  miles  per  second. 

If  the  F-line  were  to  suffer  an  equal  displacement  to  the 
left,  that  is  to  say,  towards  the  red,  the  wave-length  of  the 
greenish-blue  hydrogen  light  would  then  be  lengthened  ;  the 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES. 


127 


gas  would  therefore  be  moving  away  from  the  earth  at  the 
same  rate  of  thirty-six  miles  in  a  second,  and  the  stream  of 
gas  be  sinking  down  to  the  surface  of  the  sun. 

Fig.  XL  shows  the  displacement  which  occurs  when  a 
cyclone  takes  place  on  the  sun's  surface.  Such  a  circular 
storm  or  cyclone  was  observed  by  Lockyer  on  the  sun's 
limb,  on  the  I4th  of  March,  1869.  With  the  first  setting  ot 

FIG.  XL 


MOVEMENT  OF  A  GAS-VORTEX   IN   THE  SUN. 

the  slit,  the  image  of  the  bright  F-line  (H  |?)  in  the  chromo- 
sphere appeared  in  the  spectroscope,  as  in  No.  I  ;  a  slight 
alteration  of  the  slit  gave  in  succession  the  pictures  2  and  3. 
There  occurred  also  a  simultaneous  displacement  of  the 
bright  F-line  towards  both  the  red  and  violet  —  a  sign  that 
at  that  place  on  the  sun  a  portion  of  the  hydrogen  was  mov- 
ing towards  the  earth,  while  another  portion  was  going  in 
an  opposite  direction  away  from  the  earth  towards  the  sun, 
and  thus  the  whole  action  of  the  gas  in  motion  resembled 
that  of  a  whirlwind. 

By  means  of  the  distances  from  the  normal  dark  F-line 
which  are  taken  from  Angstrom's  maps,  Lockyer  found  that 
the  furthest  displacement  of  the  bright  F-line  corresponded 
to  a  shortening  of  the  wavelength  that  indicated  a  velocity 
in  the  stream  of  gas  of  at  least  one  hundred  and  forty-seven 

33 


128  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHEME*. 

miles  in  a  second  in  the  direction  from  the  sun  towards  the 
earth. 

These  spectroscopic  observations  receive  an  additional 
interest  when  taken  in  connection  with  those  made  with  the 
telescope.  On  the  2ist  of  April,  1869,  Lockyer  observed  a 
spot  in  the  neighborhood  of  the  sun's  limb.  At  7  h.  30  m.,  a 
prominence  showing  great  activity  appeared  in  the  field  of 
view.  The  lines  of  hydrogen  were  remarkably  brilliant, 
and  as  the  spectrum  of  the  spot  was  visible  in  the  same 
field,  it  could  be  seen  that  the  prominence  was  advancing 
towards  the  spot.  The  violence  of  the  eruption  was  so 
great  as  to  carry  up  a  quantity  of  metallic  vapors  out  of  the 
photosphere  in  a  manner  not  previously  observed.  High  up 
in  the  flame  of  hydrogen  floated  a  cloud  of  magnesium  va- 
por. At  8  h.  30  m.  the  eruption  was  over ;  but  an  hour 
later  another  eruption  began,  and  the  new  prominence  dis- 
played a  motion  of  extreme  rapidity.  Whilst  this  was  tak- 
ing place,  the  hydrogen  lines  at  the  side  of  the  spot  nearest 
to  the  earth  were  suddenly  changed  into  bright  lines,  and 
expanded  so  remarkably  as  to  give  undoubted  evidence  of 
the  occurrence  of  a  cyclonic  storm. 

The  sun  was  photographed  at  Kew  on  the  same  day  at 
10  h.  55  m. ;  the  picture  showed  clearly  that  great  disturb- 
ances had  taken  place  in  the  photosphere  in  the  neighbor- 
hood of  the  spot  observed  by  Lockyer.  In  a  second  photo- 
graph, taken  at  4  h.  i  m.,  the  sun's  limb  appeared  as  if  torn 
away  just  at  the  place  where  the  spectroscope  had  revealed 
a  rotatory  storm. 

Who  could  have  dreamed,  ten  years  ago,  that  we  should 
so  soon  attain  such  an  insight  into  the  processes  of  creation? 
And  yet,  great  though  the  results  of  spectrum  analysis  al- 
ready are,  they  are  but  a  tithe  of  the  numerous  questions 
which  this  branch  of  discovery  has  opened  up,  —  questions 
of  such  number  and  magnitude,  that  many  generations  of 
men  will  pass  away  before  they  are  all  satisfactorily  an- 
swered. 

34 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  129 

SPECTRA  OF  THE  MOON  AND  PLANETS. 

Since  the  planets  and  their  satellites  do  not  emit  light  of 
their  own,  but  shine  only  by  the  reflected  light  of  the 
sun,  their  spectra  are  the  same  as  the  solar  spectrum,  and 
any  differences  that  may  be  perceived  can  arise  only  from 
the  changes  the  sunlight  may  undergo  by  reflection  from  the 
surfaces  of  these  bodies,  or  by  its  passage  through  their 
atmospheres. 

The  observations  of  Huggins  and  Miller,  as  well  as  Jans- 
sen,  agree  in  establishing  the  complete  accordance  of  the 
lunar  spectrum  with  that  of  the  sun.  In  all  the  various  por- 
tions of  the  moon's  disk  brought  under  observation,  no  dif- 
ference could  be  perceived  in  the  dark  lines  of  the  spectrum, 
either  in  respect  of  their  number  or  relative  intensity.  From 
this  entire  absence  of  any  special  absorption  lines,  it  must  be 
concluded  that  there  is  no  atmosphere  in  the  moon,  —  a  con- 
clusion previously  arrived  at  from  the  circumstance  that 
during  an  occultation  no  refraction  is  perceived  on  the 
moon's  limb  when  a  star  disappears  behind  the  disk. 

The  spectra  of  the  planets  Venus,  Mars,  Jupiter,  and 
Saturn  are  also  characterized  by  the  Fraunhofer  lines  pe- 
culiar to  the  solar  light,  but  contain,  in  addition,  the  absorp- 
tion lines  which  are  known  to  be  telluric  lines,  and  are  evi- 
dence of  the  presence  of  an  atmosphere  containing  aqueous 
vapor. 

The  spectrum  of  Jupiter,  which  has  been  recently  exam- 
ined by  Browning  with  a  spectroscope  attached  to  his  1 2£- 
inch  reflector,  is  not  of  sufficient  brilliancy  to  allow  of  its 
being  observed  or  measured  with  extreme  accuracy.  Not- 
withstanding the  great  brilliancy  with  which  this  planet 
shines  in  the  heavens,  its  spectrum  is  not  so  bright  as  that 
of  a  star  of  the  second  magnitude ;  this  is  owing  to  the 
brightness  being  more  apparent  than  real,  and  arises  from 
the  large  size  of  the  disk  compared  with  a  star,  and  from 
the  light  being  reflected,  and  not  original. 

35 


130  RECREATIONS   IN   POPULAR   SCIENCE.  [SCHELLEN. 

The  comparatively  faint  spectrum  of  Saturn  has  been  ex- 
amined by  Huggins,  who  observed  in  it  some  of  the  lines 
characteristic  of  Jupiter's  spectrum.  These  lines  are  less 
clearly  seen  in  the  light  of  the  ring  than  in  that  of  the  ball, 
whence  it  may  be  concluded  that  the  light  from  the  ring 
suffers  less  absorption  than  does  the  light  from  the  planet 
itself.  The  observations  of  Janssen,  which  have  been  sup- 
ported by  Secchi,  have  since  shown  that  aqueous  vapor  is 
probably  present  both  in  Jupiter  and  Saturn.  Secchi  has 
further  discovered  some  lines  in  the  spectrum  of  Saturn 
which  are  not  coincident  with  any  of  the  telluric  lines,  nor 
with  any  of  the  lines  of  the  solar  spectrum  produced  by  the 
aqueous  vapor  of  the  earth's  atmosphere.  It  is  not  improb- 
able, therefore,  that  the  atmosphere  of  Saturn  may  contain 
gases  or  vapors  which  do  not  exist  in  that  of  our  earth. 

The  spectrum  of  Uranus,  which  has  been  investigated 
by  Secchi,  appears  to  be  of  a  very  remarkable  character. 
It  consists  mainly  of  two  broad  black  bands,  one  in  the 
greenish-blue,  but  not  coincident  with  the  F-line,  and  the 
other  in  the  green,  near  the  line  E.  A  little  beyond  the 
band  the  spectrum  disappears  altogether,  and  shows  a 
blank  space  extending  entirely  over  the  yellow  to  the  red, 
where  there  is  again  a  faint  re-appearance  of  light.  The 
spectrum  is  therefore  such  a  one  as  would  be  produced  were 
all  the  yellow  rays  extinguished  from  the  light  of  the  sun. 
The  dark  sodium  line  D  occurs,  as  is  well  known,  in  the 
part  of  the  spectrum  occupied  by  this  broad,  non-luminous 
space :  is  this  extraordinary  phenomenon,  therefore,  to  be 
ascribed  to  the  influence  of  this  metal,  or  is  the  planet  Ura- 
nus, which  has  a  spectrum  differing  so  greatly  from  that  of 
the  sun,  self-luminous?  Has  the  planet  not  yet  attained  that 
degree  of  consistency  possessed  by  the  nearer  planets,  which 
shine  only  by  the  sun's  light,  and,  as  the  photometric  obser- 
vations of  Zollner  lead  us  to  suppose  is  possible,  is  still  in 
that  process  of  condensation  and  subsequent  development 
through  which  the  earth  has  already  passed?  These  are 

36 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  131 

questions  to  which,  at  present,  we  can  furnish  no  reply,  and 
the  problem  can  only  be  solved  by  additional  observations 
of  the  strange  characteristics  exhibited  by  this  spectrum. 

While  Jupiter  and  his  satellites,  with  a  power  of  350,  give 
a  sharply  defined  image,  the  disk  of  Neptune,  with  the  same 
power,  ceases  to  be  well  defined,  and  appears  with  a  neb- 
ulous edge.  From  this  it  may  be  inferred  that  the  planet  is 
surrounded  by  a  dense  mist  of  considerable  extent,  the 
chemical  nature  of  which  has  yet  to  be  discovered,  or  else 
that,  like  Jupiter,  Saturn,  and  Uranus,  it  has  not  yet  attained 
that  degree  of  density  which  must  necessarily  precede  the 
formation  of  a  solid  surface. 


SPECTRA  OF  THE  FIXED  STARS. 

The  fixed  stars,  though  immensely  more  remote,  and  less 
conspicuous  in  brightness  than  the  moon  and  planets,  yet 
from  the  fact  of  their  being  original  sources  of  light,  fur- 
nish us  with  fuller  indications  of  their  nature.  In  all  ages, 
and  among  every  people,  the  stars  have  been  the  object  of 
admiring  wonder,  and  not  unfrequently  of  superstitious  ado- 
ration. The  greatest  investigators  and  the  deepest  thinkers 
who  have  devoted  themselves  to  the  study  of  the  stars,  have 
felt  a  longing  to  know  more  of  these  sparkling  mysteries. 

The  telescope  has  been  appealed  to,  but  in  vain,  for  in  the 
largest  instruments  the  stars  remained  diskless,  never  ap- 
pearing more  than  as  brilliant  points.  The  stars  have  indeed 
been  represented  as  suns,  each  surrounded  by  a  dependent 
group  of  planets,  but  this  opinion  rested  only  upon  a  possible 
analogy,  for  of  the  peculiar  nature  of  these  points  of  light, 
and  of  what  substances  they  are  composed,  the  telescope 
yields  us  no  information.  Spectrum  analysis  alone  can  dis- 
close to  us  this  much-coveted  knowledge,  as  it  gives  us  the 
means  of  reading,  in  the  light  emitted  by  these  heavenly 
bodies,  the  indications  of  their  true  nature  and  physical  con- 
stitution. In  this  light  we  possess  a  telegraphic  communi- 

37 


132  RECREATIONS  IN  POPULAR  SCIENCE.     [SCHELLKN. 

cation  between  the  stars  and  our  earth ;  the  spectroscope  is 
the  telegraph,  the  spectrum  lines  are  individually  the  letters 
of  the  alphabet,  their  united  assemblage  as  a  spectrum  forms 
the  telegram.  It  is  not,  however,  easy  to  comprehend  this 
language  of  the  stars,  but  through  the  indefatigable  labors 
of  Secchi,  Huggins,  and  Miller,  most  of  the  bright  stars,  the 
nebula?,  and  some  of  the  comets  have  been  investigated  by 
spectrum  analysis,  and  valuable  evidence  obtained  as  to  their 
physical  constitution. 

As  the  spectra  of  the  stars  bear  in  general  a  marked  re- 
semblance to  the  spectrum  of  the  sun,  being  continuous,  and 
crossed  by  dark  lines,  there  is  every  reason  for  applying 
KirchhofF's  theory  also  to  the  fixed  stars,  and  for  accepting 
the  same  explanation  of  these  similar  phenomena  that  we 
have  already  accepted  for  the  sun.  By  the  supposition  that 
the  vaporous  incandescent  photosphere  of  a  star  contains  or 
is  surrounded  by  heated  vapors,  which  absorb  the  same  rays 
of  light  which  they  would  emit  when  self-luminous,  we  may 
discover  from  the  dark  lines  in  the  stellar  spectra  the  sub- 
stances which  are  contained  in  the  photosphere  or  atmos- 
phere of  each  star.  In  order  to  ascertain  this  with  certainty, 
the  dark  lines  must  be  compared  with  the  bright  lines  of 
terrestrial  substances  volatilized  in  the  electric  spark ;  and 
the  complete  coincidence  of  the  characteristic  bright  lines 
of  a  terrestrial  substance  with  the  same  number  of  dark 
lines  in  the  stellar  spectrum,  would  justify  the  conclusion 
that  this  substance  is  present  in  the  atmosphere  of  the  star 
—  a  conclusion  that  gains  all  the  more  in  certainty,  the 
greater  the  number  of  lines  coincident  in  the  two  spectra. 

The  fact  that  certain  stars  possess  an  atmosphere  of  aque- 
ous vapor  has  been  observed  both  byjanssen  and  Secchi. 
They  belong,  for  the  most  part,  to  the  class  of  red  and  yel- 
low stars,  and  in  their  spectra,  as  might  be  supposed,  the 
lines  of  luminous  hydrogen  are  wanting.  As  early  as  1864, 
Janssen  had  remarked  the  existence  of  an  atmosphere  of 
aqueous  vapor  in  the  star  Antares ;  and  after  a  more  com- 

33 


SPECTRUM  ANALYSIS  DISCOVERIES.  133 

plete  investigation  of  the  spectrum  of  steam  in  1866,  and 
further  observations  of  stellar  spectra,  made  after  the  total 
solar  eclipse  of  1868  in  the  remarkably  dry  air  of  the  heights 
of  Sikkim  (Himalaya),  he  could  no  longer  doubt  that 
there  are  many  stars  surrounded  by  a  similar  atmosphere. 
Notwithstanding  the  dry  condition  of  the  air,  the  lines  of 
aqueous  vapor  were  more  strongly  marked  in  the  spectra  of 
these  stars,  as  seen  from  the  heights  of  the  Himalaya,  than 
had  been  observed  previously  —  a  phenomenon  which  can- 
not be  ascribed  to  the  absorption  of  the  earth's  atmosphere, 
and  must  therefore  be  due  to  that  of  the  star. 

From  all  the  observations  thus  far  made,  it  may  be  con- 
cluded that  at  least  the  brightest  stars  have  a  physical  consti- 
tution similar  to  that  of  our  sun.  Their  light  radiates,  like 
that  of  the  sun,  from  matter  in  a  state  of  intense  incandes- 
cence, and  passes,  in  like  manner,  through  an  atmosphere 
of  absorptive  vapors.  Notwithstanding  this  general  con- 
formity of  structure,  there  is  yet  a  great  difference  in  the 
constitution  of  individual  stars  ;  the  grouping  of  the  various 
elements-  is  peculiar  and  characteristic  for  each  star,  and  we 
must  suppose  that  even  these  individual  peculiarities-  are  in 
necessary  accordance  with  the  special  object  of  the  star's 
existence,  and  its  adaptation  to  the  animal  life  of  the  plan- 
etary worlds  by  which  it  is  surrounded. 

COLOR  OF  THE  STARS.  —  DOUBLE   STARS  AXD  THEIR 
SPECTRA. 

In  a  transparent  atmosphere,  especially  in  a  southern 
clime,  the  stars  do  not  all  appear  with  the  white  brilliancy 
of  the  diamond :  here  and  there  the  eye  discovers  richly- 
colored  gems,  sparkling  on  the  sombre  robe  of  night,  in 
every  shade  of  red,  green,  blue,  and  violet;  and  the  as- 
tronomer, enabled  by  his  powerful  telescope  to  investigate 
the  faintest  objects,  is  lost  in  wonder  over  the  variety  of 
these  colors,  and  their  remarkable  distribution  in  the  starry 

39 


134  RECREATIONS   IN   POPULAR   SCIENCE.  [SCHELLBN. 

heavens.  This  play  of  color  is  most  conspicuous  in  the 
double  stars,  so  called  from  their  consisting  of  two  or  more 
suns,  kept  together  by  the  bond  of  mutual  attraction,  and 
revolving  in  orbits  according  to  their  mass,  either  one  around 
the  other  or  both  round  a  common  centre  of  gravity.  To 
the  naked  eye  their  appearance  is  that  of  a  single  star,  on 
account  of  their  close  proximity ;  but  on  the  application  of 
sufficient  magnifying  power,  they  are  found  to  be  constituted 
of  three,  four,  or  more  suns  in  intimate  connection :  such  a 
system  is  to  be  found  in  the  beautiful  constellation  of  Orion 
(in  the  Sword),  consisting  of  sixteen  stars,  where  to  the  un- 
assisted eye  there  seems  but  one.  In  several  of  these  double 
stars,  the  number  of  which  already  exceeds  six  thousand,  it 
has  been  possible  to  calculate  the  time  of  revolution  of  the 
small  star.  The  period  of  one  in  the  Great  Bear  has  been 
found  to  be  sixty  years ;  of  another,  in  Virgo,  five  hundred 
and  thirteen  years ;  and  of  y  Leonis  twelve  hundred  years. 

A  peculiar  interest  attaches  to  double  stars  from  their 
great  diversity  of  color,  which  occasioned  Sir  John  Herschel 
to  remark,  in  describing  a  cluster  in  the  Southern  Cross,  that 
it  resembled  a  splendid  ornament  composed  of  the  richest 
jewels.  While  the  majority  of  single  stars  shine  with  a 
white  light,  but  sometimes  with  a  yellow,  and  even  occasion- 
ally with  a  red  hue,  in  double  stars  the  companion  is  almost 
always  blue,  green,  or  red,  thus  contrasting  with  the  white 
light  of  the  larger  or  central  star. 

It  has  long  been  a  subject  of  inquiry  whence  these  colors 
arise.  It  has  been  supposed  that  they  were  complementary 
colors,  and  therefore  that  they  were  not  inherent  in  the  stars, 
but  dependent  on  an  optical  illusion  similar  to  that  produced 
by  looking  upon  a  white  wall  immediately  after  gazing  at 
the  sun,  when  the  wall  appears  covered  with  violet  spots. 
But  the  simple  expedient  of  covering  the  central  star  in  the 
telescope  suffices  to  show  the  incorrectness  of  this  supposi- 
tion :  for  the  color  of  the  small  star  remains  unaffected  by 
its  separation  from  the  light  of  the  larger  one.  Zollner,  to 

40 


SCHBLLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  135 

whom  we  are  indebted  for  a  masterly  work  on  light  and 
the  physical  constitution  of  the  heavenly  bodies,  was  the  first 
to  express  the  idea  that,  as  all  known  substances,  in  their 
transition  from  a  state  of  incandescence  to  that  of  a  lower 
temperature,  pass  through  the  stage  of  red  heat,  so  the  fixed 
stars,  in  their  process  of  development  from  the  condition  of 
glowing  gas  through  the  period  of  an  incandescent  liquid 
state,  and  the  subsequent  development  of  floating  scoriaB,  or 
gradual  formation  of  a  cold,  non-luminous  surface,  must, 
together  with  the  gradual  diminution  of  their  light,  be  also 
subject  to  a  change  of  color.  For  many  colored  stars,  espe- 
cially for  the  so-called  new  stars,  in  which  the  color  has 
been  known  to  sink  in  the  scale  from  white  to  yellow  and  to 
red,  this  conjecture  of  Zollner's  has  a  high  degree  of  prob- 
ability ;  but  that  other  circumstances  must  exercise  an  influ- 
ence also  on  the  color  of  stars,  is  proved  by  a  change  of 
color  having  been  observed  to  take  place  in  the  opposite 
direction, — that  is,  from  red  to  white,  —  of  which,  among 
other  stars,  we  have  an  example  in  Sirius,  regarded  by  the 
ancients  as  a  red  star,  and  which  is  now  considered  as  a  type 
of  the  white  stars,  as  well  as  in  Capella,  which  formerly 
was  red,  and  now  shines  with  a  pale  blue  light.  Huggins 
and  Miller  have  discovered,  by  means  of  the  spectroscope, 
that  the  color  of  a  star  not  only  depends  upon  the  degree  of 
incandescence  of  the  intensely  hot  liquid  or  solid  nucleus, 
but  also  upon  the  kind  of  absorptive  power  its  atmosphere 
may  exert  upon  the  light  emitted  by  the  glowing  nucleus. 

As  the  source  of  stellar  light,  remarks  Huggins,  is  incan- 
descent solid  or  liquid  matter,  it  appears  very  probable  that 
at  the  time  of  its  emission  the  light  of  all  stars  is  alike 
white. ,  The  colors  in  which  we  see  them  must  'therefore  be 
produced  by  certain  changes  which  the  light  has  undergone 
since  its  emission.  It  is  further  obvious  that  if  the  dark  ab- 
sorption lines  are  more  numerous,  or  more  strongly  marked 
in  some  parts  of  the  spectrum  than  in  others,  then  the  pe- 
culiar colors  of  those  places  will  be  subdued  in  tone,  and  in 

41 


136  RECREATIONS  IN  POPULAR  SCIENCE.     [SCHELLEN. 

any  case  will  appear  relatively  weaker  than  in  those  parts 
of  the  spectrum  where  the  absorption  lines  are  much  less 
numerous.  While  in  this  way  certain  colors  would  be  par- 
tially extinguished  from  the  spectrum,  the  remaining  colors, 
being  unaffected,  would  predominate,  and  give  their  own 
tints  to  the  originally  white  light  of  the  star. 

The  colors  of  the  stars  are,  therefore,  without  doubt  pro- 
duced by  the  vapors  of  certain  substances  contained  in  their 
atmosphere ;  and  as  the  chemical  constitution  of  the  atmos- 
phere of  a  star  depends  upon  the  elements  of  which  the  star 
itself  is  composed,  and  upon  its  temperature,  it  would  be 
possible  to  ascertain  the  chief  constituents  of  these  small 
telescopic  worlds,  if  the  position  of  the  dark  absorption  lines 
could  be  determined  with  accuracy,  or  if  these  lines  could 
be  compared  with  the  spectrum  lines  of  terrestrial  elements. 

VARIABLE  STARS. 

Among  the  fixed  stars  there  are  several  which  vary,  from 
time  to  time,  in  brightness,  as  compared  with  neighboring 
stars ;  their  light  increases  or  diminishes,  and  alternates,  in 
some  cases,  from  the  brilliancy  even  of  a  star  of  the  first 
magnitude  to  complete  invisibility.  In  some,  this  change  of 
brightness  takes  place  as  a  constant,  very  slow  and  regular 
diminution  of  light ;  in  others,  there  appears  an  almost  sud- 
den increase  and  decrease  of  brilliancy ;  while  with  others, 
again,  the  change  takes  place  within  regularly  recurring 
periods.  The  period  of  variability  is,  therefore,  the  time 
elapsing  between  the  two  successive  seasons  of  greatest 
brilliancy. 

Of  all  variable  stars,  Mira  Ceti  is  perhaps  the  most  inter- 
esting, since,  at  its  maximum  brightness,  it  equals  a  star  of 
the  first  or  second  magnitude.  Scarcely  less  interesting  is 
(9  Persei,  which,  for  two  days  thirteen  hours  and  a  half, 
shines  with  the  brightness  of  a  star  of  the  second  magnitude, 
then  suddenly  decreases  in  light,  and  sinks  down,  in  three 

42 


SCHELLEN.]     SPECTRUM   ANALYSIS   DISCOVERIES.  137 

hours  and  a  half,  to  a  star  of  the  fourth  magnitude ;  its  light 
then  again  increases,  and  in  a  similar  period  of  three  hours 
and  a  half  regains  its  original  brilliancy.  All  these  changes 
recur  regularly  in  the  space  of  less  than  three  days,  during 
which  the  star  always  remains  visible  to  the  naked  eye. 

Whence  comes  this  variation  in  the  light  of  a  star?  Zoll- 
ner,  with  great  acuteness,  and  supported  by  numerous  obser- 
vations of  these  changes  of  brightness,  offers  a  simple  and 
unconstrained  explanation,  in  supposing  the  cause  to  lie  in 
the  configuration  and  distribution  of  dark  masses  of  scoriae, 
which  form  on  the  red-hot  liquid  body  of  the  star  in  the 
process  of  cooling,  and  which,  in  consequence  of  the  star's 
rotation  on  its  axis,  and  the  centrifugal  force  thus  arising, 
would  take  certain  definite  courses  on  the  surface  of  the  star 
in  a  manner  analogous  to  that  which  may  be  observed  with 
floating  icebergs  on  our  earth.  As  a  consequence  of  this 
peculiar  relative  motion,  the  dark  masses  of  scorias  would 
arrange  themselves  in  a  fixed  order,  and  would  produce 
on  the  surface  of  the  star  an  unequal  distribution  of  red- 
hot  luminous  matter,  and  accumulations  of  non-luminous 
scoriaB. 

It  has  recently  been  remarked  by  Secchi  that  the  spectrum 
of  the  nucleus  of  a  solar  spot  bears  a  close  resemblance  to 
that  given  by  several  red  stars,  such  as  «  Orionis,  Antares, 
Aldebaran,  o  Ceti.  A  series  of  dark  bands  and  stripes,  as 
represented  in  the  spectrum  of  «  Orionis,  are  present  equally 
in  the  spectrum  of  a  solar  spot  as  in  the  spectra  of  the  above 
named  red  stars,  which  leads  to  the  supposition  that  the  red 
color  of  these  stars  arises  from  the  same  cause  that  produces 
the  absorption  bands  in  the  spectrum  of  the  solar  spot.  As 
nearly  all  these  stars  are  variable,  it  is  not  improbable  that 
they  are  also  subject  to  spots  which  occur  with  a  certain  de- 
gree of  regularity,  as  the  solar  spots  have  been  proved  to  do. 
The  period  of  variability  in  the  light  would  then  depend 
upon  the  period  of  the  formation  of  the  spots,  in  the  same 
way  as  our  sun  appears  as  a  variable  star,  of  which  the 

43 


138  RECREATIONS   IN   POPULAR   SCIENCE.   [SCHELLEN. 

period  of  variation  in  the  light  coincides  with  the  regular 
recurrence  of  the  spots. 


NEW  OR  TEMPORARY  STARS. 

Among  the  variable  stars  must  also  be  reckoned  those 
which,  from  time  to  time,  but  only  at  exceedingly  long  in- 
tervals, have  suddenly  flamed  forth  in  the  sky  and  disap- 
peared again  after  a  longer  or  shorter  interval,  and  which 
always  excite  the  greatest  wonder  and  interest,  not  only  from 
the  rarity  of  their  appearance,  but  also  from  the  mighty  rev- 
olutions in  space  which  they  announce.  According  to  Hum- 
boldt,  only  twenty-one  such  stars  have  been  recorded  in  the 
space  of  two  thousand  years,  —  from  134  B.  C.  to  1848  A.  D., 
—  the  most  remarkable  of  which  was  that  observed  by  Ty- 
cho  Brahe  (1572)  in  Cassiopeia?,  which  surpassed  both  Sirius 
and  Jupiter,  and  even  rivalled  Venus  in  brilliancy,  but  dis- 
appeared after  seventeen  months,  without  leaving  a  trace 
visible  to  the  naked  eye;*  and  that  seen  by  Kepler  (1604) 
in  the  right  foot  of  Ophiuchus,  which  excelled  Jupiter,  but 
did  not  quite  equal  Venus,  in  brightness,  and  at  the  end  of 
fifteen  months  was  visible  only  by  means  of  the  telescope. 
Two  similar  stars,  which  have  appeared  in  recent  times, — 
one  observed  by  Hind,  in  1848,  and  another  seen  in  the 
Northern  Crown,  in  1866,  —  though  they  soon  lost  their 
ephemeral  glory,  still  continue  visible  as  stars  of  the  tenth 
and  ninth  magnitude.  A  characteristic  peculiarity  of  these 
temporary  stars  is,  that  they  nearly  all  flash  out  at  once  with 
a  degree  of  brilliancy  exceeding,  in  some  cases,  even  stars 
of  the  first  magnitude,  and  that  they  have  not  been  ob- 
served, at  least  with  the  naked  eye,  to  increase  gradually  in 
brightness. 

Are  we  to  suppose  that  these  so-called  new  stars  are  really 

*  The  telescope  was  not  invented  until  thirty-seven  years  aftef 
this  date. 

44 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES'.  139 

new  creations,  as  Tycho  Brahe  believed,  and  that  those  that 
have  disappeared  are  really  annihilated  or  burned  out?  Can 
we  suppose,  with  Riccioli,  that  these  heavenly  bodies  are 
luminous  only  on  one  side,  which  by  a  sudden  semi-revolu- 
tion the  Creator,  at  the  appointed  time,  has  turned  towards 
us  ?  The  first  supposition  has  been  set  aside  by  later  obser- 
vations, which  have  shown,  by  the  help  of  maps,  that  a 
small  star  had  already  existed  precisely  in  the  place  where 
the  new  star  burst  forth  ;  the  other  view  is  too  absurd  to  de- 
serve, in  these  days,  any  further  consideration.  The  star 
observed  by  Tycho,  as  well  as  that  one  seen  by  Kepler,  are 
still  visible.  If,  therefore,  the  sudden  bursting  forth  of  a 
star  in  the  heavens  does  not  denote  the  creation  of  a  new 
star,  nor  its  gradual  disappearance  indicate  its  complete 
annihilation,  we  may  well  suppose  that  both  phenomena  are 
the  successive  effects  of  a  violent  outbreak  of  fire  taking 
place  in  the  star,  either  in  the  form  of  an  eruption  of  the 
internal  red-hot  liquid  matter,  and  its  suffusion  over  the  sur- 
face, or  of  the  ignition  of  gigantic  streams  of  gas  forcing 
their  way  from  the  interior.  While  such  an  occurrence 
would  raise  the  star  to  a  state  of  extreme  incandescence,  and 
cause  it  to  emit  an  intense  light  for  some  time,  the  cooling 
subsequent  to  this  combustion  would  ensue  more  or  less  rap- 
idly, and  the  brightness  consequently  diminish  in  quick  pro- 
gression, until,  in  certain  conditions,  the  star  would  cease  to 
be  visible. 

Fortunately  for  science,  such  an  occurrence  has  taken 
place  since  spectrum  analysis  has  been  so  successfully  ap- 
plied to  the  examination  of  the  heavenly  bodies.  On  the 
night  of  the  I2th  of  May,  1866,  a  new  star,  brighter  than 
one  of  the  second  magnitude,  was  observed  at  Tuam,  by 
Mr.  John  Birmingham,  in  the  constellation  Corona  Borealis. 
On  the  following  night  it  was  seen  by  the  French  engineer, 
Courbebaisse,  at  Rochefort,  and  was  observed  a  few  hours 
earlier  at  Athens  by  the  astronomer  Julius  Schmidt,  who 
expressly  declares  that  the  new  star  could  not  have  been  vis* 

45 


140  RECREATIONS   IN  POPULAR   SCIENCE. 

ible  before  eleven  o'clock  on  the  night  of  the  I2th  of  May, 
as  he  had  been  observing  with  his  comet-seeker  the  star 
R  Coronae,  and  while  sweeping  for  some  time  in  its  neigh- 
borhood for  meteors,  could  not  have  failed  to  notice  the  new 
star,  if  it  had  been  then  visible.  On  the  same  night  (i3th 
of  May)  the  light  of  the  star  sensibly  decreased,  and  by  the 
1 6th  of  May  it  had  become  only  of  the  fourth  magnitude. 
Its  brightness  then  waned  somewhat  rapidly :  it  decreased 
from  4.9  on  the  i7th  to  5.3  on  the  iSth,  and  from  5.7  ou  the 
1 9th  to  6.2  on  the  2oth,  till,  by  the  end  of  the  month,  it  had 
become  a  star  of  the  ninth  magnitude. 

Argelander  observed  the  star  on  the  iSth  of  May,  1855, 
and  on  the  3ist  of  March,  1856,  and  on  both  occasions  had 
classed  the  star  as  between  the  ninth  and  tenth  magnitudes. 

Huggins  was  informed  by  Birmingham  of  his  discovery 
on  the  I4th  of  May,  and  was  thus  enabled,  on  the  I5th  inst, 
in  conjunction  with  Miller,  to  examine  the  spectrum  of  this 
star  when  it  had  not  fallen  much  below  the  third  magnitude. 
The  result  of  this  investigation  is  as  follows. 

The  spectrum  of  the  star  was  very  remarkable,  and 
showed  clearly  that  there  were  two  distinct  sources  of  light, 
each  producing  a  separate  spectrum,  —  one  a  continuous 
spectrum,  crossed  by  dark  lines  similar  to  that  given  by  the 
sun  and  other  stars,  while  the  other  consists  of  four  bright 
lines,  which,  from  their  great  brilliancy,  stand  in  bold  relief 
upon  the  dark  background  of  the  first  spectrum. 

The  principal  spectrum  traversed  by  dark  lines  shows  the 
presence  of  a  photosphere  of  incandescent  matter,  probably 
solid  or  liquid,  which  is  surrounded  by  an  atmosphere  of 
cooler  vapors,  giving  rise  by  absorption  to  the  dark  lines. 
This  absorption  spectrum  contains  two  strong  dark  bands, 
of  less  refrangibility  than  the  D-line  of  the  solar  spectrum  ; 
a  group  of  fine  lines  stretches  from  them  close  up  to  D, 
while  one  fine  line  is  quite  coincident  with  D.  Up  to  this 
point,  the  constitution  of  this  object  is  analogous  to  that  of 
the  sun  and  the  stars;  but  the  star  has  also  a  spectrum  con- 


SCHELLEX.]     SPECTRUM  ANALYSIS  DISCOVERIES.  141 

sisting  of  bright  lines,  which  denotes  the  presence  of  a  sec- 
ond source  of  light,  which,  from  the  nature  of  the  spectrum, 
is  undoubtedly  an  intensely  luminous  gas. 

Huggins  compared  the  spectrum  of  the  star  on  the  lyth 
of  May  with  the  spectrum  of  hydrogen  gas  produced  by 
means  of  the  induction  spark  through  a  GeiSvsler's  tube,  and 
found  that  the  strongest  of  the  stellar  lines  2  was  coincident 
with  the  greenish-blue  line  H  ^  of  hydrogen  gas.  Appar- 
ently, also,  the  line  i  in  the  red  coincided  with  the  H  "-line 
of  hydrogen,  but  owing  to  the  want  of  brilliancy  of  the  line, 
the  coincidence  could  not  be  ascertained  with  the  same  de- 
gree of  certainty.  The  great  brilliancy  of  these  lines,  com- 
pared with  the  parts  of  the  continuous  spectrum  where  they 
occur,  proves  that  the  luminous  gas  was  at  a  higher  temper- 
ature than  the  photosphere  of  the  star. 

These  facts,  taken  in  connection  with  the  suddenness  of 
the  outburst  of  light  in  the  star,  and  the  immediate  very 
rapid  decline  in  its  brightness  from  the  second  down  to  the 
eighth  magnitude,  have  led  to  the  hypothesis  already  alluded 
to,  that,  in  consequence  of  some  internal  convulsion,  enor- 
mous quantities  of  hydrogen  and  other  gases  were  evolved, 
which,  in  combining  with  some  other  elements,  ignited  on 
the  surface  of  the  star,  and  thus  enveloped  the  whole  body 
suddenly  in  a  sheet  of  flame.  The  ignited  hydrogen  gas,  in 
its  combination  with  some  other  element,  produced  the  light 
characterized  by  the  two  bright  bands  in  the  red  and  green  ; 
the  remaining  bright  lines,  among  which  those  of  oxygen 
might  have  been  expected,  were  not  coincident  with  any  of 
the  lines  of  this  gas.  The  burning  hydrogen  gas  must  also 
have  greatly  increased  the  heat  of  the  solid  matter  of  the 
photosphere,  and  brought  it  into  a  state  of  more  intense  in- 
candescence and  luminosity,  which  may  explain  how  the 
formerly  faint  star  could  so  suddenly  assume  such  remark- 
able brilliancy.  As  the  liberated  hydrogen  gas  became  ex- 
hausted, the  flame  gradually  abated,  and  with  the  conse- 

47 


142  RECREATIONS   IN   POPULAR   SCIENCE.  [SCHB-LEH. 

quent  cooling  the  photosphere  became  less  vivid,  and  the 
star  returned  to  its  original  condition. 

Robert  Meyer  and  H.  J.  Klein  have  expressed  the  opinion 
that  the  sudden  blazing  out  of  a  star  might  be  occasioned  by 
the  violent  precipitation  of  some  great  mass,  perhaps  of  a 
planet,  upon  a  fixed  star,  by  which  the  momentum  of  the 
falling  mass  would  be  changed  into  molecular  motion,  or,  in 
other  words,  into  heat  and  light.  It  might  even  be  supposed 
that  the  star  in  Corona,  through  its  motion  in  space,  may 
have  come  in  contact  with  one  of  the  nebulae,  which  traverse 
in  great  numbers  the  realms  of  space  in  every  direction,  and 
which,  from  their  gaseous  condition,  must  possess  a  high 
temperature.  Such  a  collision  would  recessarily  set  the 
star  on  a  blaze,  and  occasion  the  most  vehement  ignition  of 
its  hydrogen. 

It  must  not  be  forgotten  that  light,  though  an  extremely 
quick  messenger,  yet  occupies  a  certain  time  ini  coming  to 
us  from  a  star.  The  speed  of  light  is  one  hundred  and 
eighty-five  thousand  miles  in  a  second.  The  distance  of  the 
nearest  fixed  star  («Centauri)  is  about  sixteen  billion  miles, 
so  that  light  takes  about  three  years  to  travel  from  this  star 
to  us.  The  great  physical  convulsion  which  was  observed 
in  the  star  in  Corona,  in  the  year  1866,  was  therefore  an 
event  which  had  really  taken  place  long  before  that  period, 
at  a  time,  no  doubt,  when  spectrum  analysis,  to  which  we 
are  indebted  for  the  information  we  obtained  on  the  subject, 
was  yet  quite  unknown. 

The  discoveries  made  by  means  of  spectrum  analysis, 
connected  with  at  least  three  classes  of  heavenly  bodies,  still 
remain  to  be  reviewed,  and  will  form  the  basis  of  another 
paper  of  this  series,  which  will  be  entitled  Nebulas,  Comets, 
and  Meteors. 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  143 


6,    $£bula3,    {gamuts, 

And  the  Revelations  of  the   Spectroscope  regarding  them. 

SPECTRA  OF  NEBULAE. 

WE  now  come  to  treat  of  the  remotest  realms  of  the 
Universe,  those  regions  of  stellar  clusters  and  neb- 
ulae which  can  only  be  reached  by  means  of  the  most  power- 
ful telescopes.  When  the  starry  heavens  are  viewed  through 
a  telescope  of  moderate  power,  a  great  number  of  stellar 
clusters  and  faint  nebulous  forms  are  revealed  against  the 
dark  background  of  the  sky,  which  might  be  taken  at  first 
sight  for  passing  clouds,  but  which,  by  their  unchanging 
forms  and  persistent  appearance,  are  proved  to  belong  to  the 
heavenly  bodies,  though  possessing  a  character  widely  dif- 
fering from  the  point-like  images  of  ordinary  stars.  Sir 
William  Herschel  was  able,  with  his  gigantic  forty-foot  tel- 
escope, to  resolve  many  of  these  nebulas  into  clusters  of  stars, 
and  found  them  to  consist  of  vast  groups  of  individual  suns, 
in  which  thousands  of  fixed  stars  may  be  clearly  separated 
and  counted,  but  which  are  so  far  removed  from  us  that  we 
are  unable  to  perceive  their  distance  one  from  the  other, 
though  that  may  really  amount  to  many  millions  of  miles, 
and  their  light,  with  a  low  magnifying  power,  seems  to  come 
from  a  large,  faintly-luminous  mass.  But  all  nebula3  were 
not  resolvable  with  this  telescope,  and  in  proportion  as  such 
nebulae  were  resolved  into  clusters  of  stars,  new  nebulas  ap- 
peared which  resisted  a  power  of  six  thousand,  and  sug- 
gested to  this  astute  investigator  the  theory  that,  besides  the 
many  thousand  apparent  nebulae  which  reveal  themselves  to 
us  as  a  complete  and  separate  system  of  worlds,  there  are 
also  thousands  of  real  nebulae  in  the  Universe,  composed  of 
primeval  cosmical  matter,  out  of  which  future  worlds 
were  to  be  fashioned. 


144  RECREATIONS   IN   POPULAR   SCIENCE.  [SCHELLEH. 

Lord  Rosse,  by  means  of  a  telescope  of  fifty-two  feet  focus, 
of  his  own  construction,  was  able  to  resolve  into  clusters  of 
stars  many  of  the  nebulae  not  resolved  by  Herschel ;  but 
there  were  still  revealed  to  the  eye,  thus  carried  farther  into 
space,  new  nebula3  beyond  the  power  even  of  this  gigantic 
telescope  to  resolve. 

Telescopes  failed,  therefore,  to  solve  the  question  whether 
the  unresolved  nebulae  are  portions  of  the  primeval  matter 
out  of  which  the  existing  stars  have  been  formed  ;  they  leave 
us  in  uncertainty  as  to  whether  these  nebulae  are  masses  of 
luminous  gas,  which,  in  the  lapse  of  ages,  would  pass  through 
the  various  stages  of  incandescent  liquid  (the  sun  and  fixed 
stars),  of  scoriae  or  gradual  formation  of  a  cold  and  non- 
luminous  surface  (the  earth  and  planets),  and  finally  of  com- 
plete gelation  and  torpidity  (the  moon),  or  whether  they 
exist  as  a  complete  and  separate  system  of  worlds ;  tele- 
scopes have  only  widened  the  problem,  and  have  neither 
simplified  nor  solved  its  difficulties. 

That  which  was  beyond  the  power  of  the  most  gigantic 
telescopes  has  been  accomplished  by  that  apparently  insig- 
nificant, but  really  delicate,  and  almost  infinitely  sensitive 
instrument,  the  spectroscope.  We  are  indebted  to  it  for 
being  able  to  say  with  certainty  that  luminous  nebulas  ac- 
tually exist  as  isolated  bodies  in  space,  and  that  these  bodies 
are  luminous  masses  of  gas. 

The  spectroscope,  in  combination  with  the  telescope,  af- 
fords means  for  ascertaining,  even  now,  some  of  the  phases 
through  which  the  sun  and  planets  have  passed  in  their  pro- 
cess of  development  or  transition  from  masses  of  luminous 
nebulae  to  their  present  condition. 

Great  variety  is  observed  in  the  forms  of  the  nebulae : 
while  some  are  chaotic  and- irregular,  and  sometimes  highly 
fantastic,  others  exhibit  the  pure  and  beautiful  forms  of  a 
curve,  a  crescent,  a  globe,  or  a  circle. 

The  largest  and  most  irregular  of  all  the  nebulae  is  that  in 
the  constellation  of  Orion  (Fig.  XII.).  It  is  situated  rather 


SCHELLEN.]     SPECTRUM  ANALYSIS   DISCOVERIES.  145 

below  the  three  stars  of  second  magnitude  composing  the 
central  part  of  that  magnificent  constellation,  and  is  visible 
to  the  naked  eye.  It  is  extremely  difficult  to  execute  even  a 
tolerably  correct  drawing  of  this  nebula ;  but  it  appears, 
from  the  various  drawings,  made  at  different  times,  that  a 

FIG.  XII. 
South. 


North. 

Central  and  most  brilliant  portion  of  the  great  Nebula  in  the  Sword-handle  of  Orion,  as  ob- 
served by  Sir  John  Herschel  in  his  20-foot  Reflector  at  Feldhausen,  Cape  of  Good  Hope 
(1834  to  1837). 

change  is  taking  place  in  the  form  and  position  of  the  bright- 
est portions.  Fig.  XII.  represents  the  central  and  brightest 
part  of  the  nebula.  Four  bright  stars,  forming  a  trapezium, 
are  situated  in  it,  one  of  which  only  is  visible  to  the  naked 
eye.  The  nebula  surrounding  these  stars  has  a  flaky  appear- 

3 


RECREATIONS   IN   POPULAR   SCIENCE.    [SCHELLEN. 

ance,  and  is  of  a  greenish-white  color;  single  portions  form 
long  curved  streaks,  stretching  out  in  a  radiating  manner 
from  the  middle  and  bright  parts. 

The  interest  aroused  by  these  irregular  and  chaotic  neb- 
ulous forms  is  still  further  increased  by  the  phenomena  of 
the  spiral  or  convoluted  nebula  with  which  the  giant  tele- 
scopes of  Lord  Rosse  and  Mr.  Bond  have  made  us  further 
acquainted.  As  a  rule,  there  streams  out  from  one  or  more 
centres  of  luminous  matter  innumerable  curved  nebulous 
streaks,  which  recede  from  the  centre  in  a  spiral  form,  and 
finally  lose  themselves  in  space. 

Fig.  XIII.  represents  the  most  remarkable  of  all  the  spiral 
nebulaB,  which  is  situated  in  the  constellation  Canes  Venatici. 

FIG.  XIII. 


SPIRAL  NEBULA   IN   CANES  VENATICI. 

It  is  hardly  conceivable  that  a  system  of  such  a  nebulous 
form  could  exist  without  internal  motion.  The  bright  nu- 
cleus, as  well  as  the  streaks  curving  round  it  in  the  same 
direction,  seem  to  indicate  an  accumulation  of  matter  towards 

4 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES. 


H7 


the  centre,  with  a  gradual  increase  of  density,  and  a  rotatory 
movement.  But  if  we  combine  with  this  motion  the  suppo- 
sition of  an  opposing  medium,  it  is  difficult  to  harmonize 

FIG.  XIV. 


TRANSITION    FROM   THE  SPIRAL  TO  THE  ANNULAR   FORM. 

such  a  system  with   the  known  laws  of  statics.     Accurate 
measures  are,  therefore,  of  the  highest  interest  for  the  pur- 

FIG.  XV. 


ANNULAR   NEBULA   IN    LYRA. 


pose  of  showing  whether  actual  rotation  or  other  changes 
are  taking  place   in  these  nebulae ;  but,  unfortunately,  they 

5 


HS  RECREATIONS  IN   POPULAR   SCIENCE.   [SCHELLEX. 

are  rendered  extremely  difficult  and  uncertain  by  the  want 
of  outline,  and  by  the  remarkable  faintness  of  these  nebulous 
objects. 

The  transition  state  from  the  spiral  to  the  annular  form  is 
shown  in  such  nebulas  as  the  one  represented  in  Fig.  XIV. ; 
and  they  then  pass  into  the  simple  or  compound  annular 
nebula,  of  which  a  type  is  given  in  Fig.  XV. 

The  space  within  most  of  these  elliptic  rings  is  not  per- 
fectly dark,  but  is  occupied  either  by  a  diffused  faint  nebulous 
light,  as  in  Fig.  XV.,  or,  as  in  most  cases,  by  a  bright  nu- 
cleus, round  which  sometimes  one  ring,  sometimes  several, 
are  disposed  in  various  forms. 

Those  nebulas  which  appear  with  tolerably  sharply-defined 

FIG.  XVI.  FIG.  XVII. 


Planetary  Nebula  with  two  Stars.  Planetary  Nebula. 

edges,  in  the  form  of  a  circle  or  slight  ellipse,  seem  to  belong 
to  a  much  higher  stage  of  development.  From  their  resem- 
blance to  those  planets  which  shine  with  a  pale  or  bluish 
light,  they  have  been  called  planetary  nebulas ;  in  form, 
however,  they  vary  considerably,  some  of  them  being  spiral 
and  some  annular.  Some  of  these  planetary  nebulas  are 
represented  in  Figs.  XVI.  and  XVII.  The  first  has  two 
central  stars  or  nuclei,  each  surrounded  by  a  dark  space, 
beyon-d  which  the  spiral  streaks  are  disposed  ;  the  second 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  149 

is   without   a    nucleus,   but   shows   a  well-defined   ring   of 
light. 

The  highest  type  of  nebulae  are  certainly  the  stellar  neb- 
ulas, in  which  a  tolerably  well  defined  bright  star  is  sur- 
rounded by  a  completely  round  disk,  or  faint  atmosphere  of 
light,  which  sometimes  fades  away  gradually  into  space,  at 
other  times  terminates  abruptly,  with  a  sharp  edge.  Fig. 
XVIII.  exhibits  one  of  the  most  striking  of  these  very  re- 
markable stellar  nebulas.  It  is  a  veritable  star  of  the  eighth 
magnitude,  and  is  not  nebulous,  but  is  surrounded  by  a 

FIG.  XVIII. 


STELLAR   NEBULA. 

bright  luminous  atmosphere  perfectly  concentric.  To  the 
right  of  the  star  is  a  small  dark  space,  such  as  often  occurs 
in  these  nebulas,  indicating,  perhaps,  an  opening  in  the  sur- 
rounding atmosphere. 

We  have  now  passed  in  review  nearly  all  that  is  at  present 
known  of  the  nebulas,  so  far  as  their  appearance  and  form 
have  been  revealed  by  the  largest  telescopes.  The  informa- 
tion as  yet  furnished  by  the  spectroscope  on  this  subject  is 
certainly  much  less  extensive,  but  is  nevertheless  of  the 

7 


150  RECREATIONS   IN  POPULAR   SCIENCE.    [SCHKLLEN. 

greatest  importance,  since  the  spectroscope  has  power  to 
reveal  the  nature  and  constitution  of  these  remote  heavenly 
bodies.  It  must  here  again  be  remembered  that  the  char- 
acter of  the  spectrum  not  only  indicates  what  the  substance 
is  that  emits  the  light,  but  also  its  physical  condition.  If 
the  spectrum  be  a  continuous  one,  consisting  of  rays  of 
every  color  or  degree  of  refrangibility,  then  the  source  of 
light  is  either  a  solid  or  liquid  incandescent  body  ;  if,  on  the 
contrary,  the  spectrum  be  composed  of  bright  lines  only, 
then  it  is  certain  that  the  light  comes  from  luminous  gas; 
finally,  if  the  spectrum  be  continuous,  but  crossed  by  dark 
lines  interrupting  the  colors,  it  is  an  indication  that  the 
source  of  light  is  a  solid  or  liquid  incandescent  body,  but 
that  the  light  has  passed  through  an  atmosphere  of  vapors  at 
a  lower  temperature,  which,. by  their  selective  absorptive 
power,  have  abstracted  those  colored  rays  which  they  would 
have  emitted  had  they  been  self-luminous. 

When  Huggins  first  directed  his  telespectroscope,  in  Au- 
gust, 1864,  to  one  of  these  objects,  a  small  but  very  bright 
nebula,  he  found,  to  his  great  surprise,  that  the  spectrum, 
instead  of  being  a  continuous  colored  band,  such  as  that 
given  by  a  star,  consisted  only  of  three  bright  lines. 

This  one  observation  was  sufficient  to  solve  the  long-vexed 
question,  at  least  for  this  particular  nebula,  and  to  prove  that 
it  is  not  a  cluster  of  individual,  separable  stars,  but  is  ac- 
tually a  gaseous  nebula,  a  body  of  luminous  gas.  In  fact, 
such  a  spectrum  could  only  be  produced  by  a  substance  in  a 
state  of  gas ;  the  light  of  this  nebula,  therefore,  was  emitted 
neither. by  solid  nor  liquid  incandescent  matter,  nor  by  gases 
.in  a  state  of  extreme  density,  as  may  be  the  case  in  the  sun 
.and  stars, 'but  *by  luminous  gas  in  a  highly  rarefied  con- 
dition. 

In  order  to  discover  the  chemical  nature  of  this  gas,  Hug- 
gins  followed  the  usual  methods  of  comparison,  and  tested 
the  spectrum  with  the  Fraunhofer  lines  of  the  solar  spectrum, 
and  the  bright  lines  of  terrestrial  elements. 

8 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  151 

Besides  the  spectrum  containing  these  three  bright  lines, 
the  nebula  gave  also  a  very  faint  continuous  spectrum,  of 
scarcely  perceptible  width,  which,  from  its  nature,  could 
proceed  only  from  the  diffused  light  of  a  faintly  glowing 
nucleus,  either  solid  or  liquid,  or  from  faintly  luminous  mat- 
ter in  the  form  of  a  cloud  of  solid  or  liquid  particles. 

All  planetary  nebulae  yield  the  same  spectrum  ;  the  bright 
lines  appear  with  considerable  intensity  in  the  spectroscope, 
and  are  of  sufficient  brilliancy  to  compare  with  the  bright 
lines  in  the  spectrum  of  a  candle,  although  the  nebulas  may 
not  be  brighter  in  the  heavens  than  stars  of  the  ninth  mag- 
nitude. The  reason  of  this  is,  that  the  light  of  the  candle  is 
spread  out  into  a  continuous  spectrum,  while  that  of  the  neb- 
ula remains  concentrated  into  a  few  lines.  The  principle  is 
identical  with  that  by  which  the  spectra  of  the  solar  prom- 
inences have  been  since  observed  in  sunlight  simultaneously 
with  the  greatly  subdued  spectrum  of  daylight. 

During  the  years  1865  and  1866,  more  than  sixty  nebulas 
were  examined  by  Huggins  with  the  spectroscope,  mainly 
with  the  intention  of  ascertaining  whether  those  which  were 
clearly  resolvable  by  the  telescope  into  a  cluster  of  bright 
points,  gave  a  continuous  spectrum,  or  one  composed  of 
bright  lines. 

As  a  result  of  his  observations,  Huggins  divides  the  neb- 
ulas into  two  groups : 

1.  The  nebulas  giving  a  spectrum  of  one  or  more  bright 
lines. 

2.  The  nebulas  giving  a  spectrum  apparently  continuous. 
About  a  third  of  the  sixty  nebulae  observed  belong  to  the 

first  group ;  their  spectrum  consists  of  one,  two,  or  three 
bright  lines ;  a  few  showing  at  the  same  time  a  very  narrow, 
faint,  continuous  spectrum. 

The  great  nebula  of  Orion  (Fig.  XII.)  has  been  the  sub- 
ject of  spectroscopic  investigations.  Its  spectrum  consists 
of  three  very  conspicuous  bright  lines,  one  of  which  again 
indicates  nitrogen,  and  another  hydrogen. 

9 


152  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLEN. 

Huggins  has  lately  repeated  his  former  observations  with 
instruments  of  much  greater  power,  and  compared  especially 
these  two  lines  with  those  of  the  terrestrial  gases,  under  cir- 
cumstances which  gave  him  a  spectrum  four  times  the  length 
of  the  one  he  obtained  in  his  earlier  investigations.  The 
result  of  these  observations,  continued  for  several  nights, 
was  to  show  the  complete  coincidence,  even  in  this  greatly 
extended  spectrum  of  the  nebular  lines,  with  those  of  both 
gases,  so  that  there  can  be  no  remaining  doubt  as  to  the 
identity  of  the  lines. 

Half  of  the  nebulae  giving  a  continuous  spectrum  have 
been  resolved  into  stars,  and  about  a  third  more  are  probably 
resolvable ;  while  of  those  yielding  a  spectrum  of  lines,  not 
one  has  been  certainly  resolved  by  Lord  Rosse.  Considering 
the  extreme  difficulty  attending  investigations  of  this  kind, 
there  is  scarcely  any  doubt  that  there  is  a  complete  accord- 
ance between  the  results  of  the  telescope  and  spectroscope ; 
and  therefore  those  nebula  giving  a  continuous  spectrum 
are  clusters  of  actual  stars,  while  those  giving  a  spectrum 
of  bright  lines  must  be  regarded  as  masses  of  luminous  gas, 
of  which  nitrogen  and  hydrogen  form  the  chief  constit- 
uents. 

COMETS  AND  THEIR  SPECTRA. 

Besides  the  planets,  which,  already  cold  or  in  process  of 
cooling,  derive  their  light  from  the  incandescent  sun,  round 
which  they  revolve  in  their  appointed  orbits,  all  travelling 
nearly  in  one  plane  among  the  fixed  stars  in  regular  progress 
from  west  to  east,  there  appear  from  time  to  time  certain 
other  wandering  stars  of  peculiar  aspect,  which,  from  their 
rapid  change  of  form  and  size,  their  fantastic  contour,  and 
their  brilliant  light,  usually  excite  the  greatest  attention; 
these  remarkable  visitors  are  comets  ;  and  though  their  laws 
of  motion  have  been  well  ascertained,  yet  their  physical  con- 
stitution has  presented  greater  difficulties  to  astronomers 
than  even  that  of  the  nebulae.  When  they  first  become  vis- 

10 


SCHHLLEN.]      SPECTRUM  ANALYSIS  DISCOVERIES.  153 

ible,  their  motion  is  evidently  round  the  £un,  but  frequently 
in  orbits  of  such  great  elongation  as  hardly  to  be  called 
elliptical,  travelling,  besides,  in  all  possible  planes  and  direc- 
tions—  sometimes,  like  the  planets,  from  west  to  east,  some- 
times in  the  reverse  way,  from  east  to  west.  Several  of 
these  extraordinary  objects  move  in  closed  orbits  round  the 
sun,  with  a  regular  period  of  revolution ;  others  come  quite 
unexpectedly  from  the  regions  of  space  into  our  system,  and 
retreat  again,  to  be  seen  no  more.  The  periodic  comets  are 
as  follows :  — 

Distance  from  the  Sun. 
Comet.  Period  Perihelion.  |  Aphelion. 

Encke's,      .  3^  years.  289,000,000  miles.        350,000,000  miles. 

Winnecke's,  5^      "  69,000,000     "  501,000,000     " 

Brorsen's,   .  5f     "  55,000,000     "  516,000,000     " 

Biela's,   .     .  6£     u  78,000,000     "  564,000,000     " 

Faye's,   .     .  i\     "  156,000,000     "  543,000,000     " 

Halley's,     .  76^     "  52,000,000     "  3,175,000,000     " 

While  these  comets  have  but  a  short  period,  there  are 
others,  such  as  the  comets  of  1858,  iSir,  and  1844,  the 
calculated  periods  of  which  amount  respectively  to  2,100, 
3,000,  and  100,000  years.  Differences  of  quite  a  proportion- 
ate magnitude  are  observable,  in  relation  to  the  points  of 
nearest  approach  to  and  greatest  distance  from  the  sun. 
Encke's  comet  is  twelve  times  nearer  the  sun  at  its  perihelion 
than  at  its  aphelion.  Some  of  them,  with  an  orbit  extending 
beyond  Jupiter,  approach  so  close  to  the  sun  as  almost  to 
graze  the  surface.  Newton  estimated  that  the  comet  of  1680 
came  so  near  to  the  sun,  that  its  temperature  must  have  ex- 
ceeded by  two  thousand  times  that  of  melted  iron.  At  its 
nearest  approach  it  was  removed  from  the  sun  by  only  a 
sixth  of  his  diameter.  The  comet  of  1843,  also,  was  so  near 
the  sun  at  its  perihelion  as  to  be  seen  in  broad  daylight. 

Most  comets  exhibit  a  planetary  disk,  more  or  less  bright, 
which  is  called  the  nucleus,  and  tTiis  is  surrounded  by  a 
fainter  cloudy  or  nebulous  envelope,  the  coma ;  the  nucleus 

ii 


154  RECREATIONS  IN  POPULAR  SCIENCE.     [SCHELLEN. 

and  coma  form  the  head  of  the  comet.  In  almost  all  comets 
visible  to  the  naked  eye,  there  streams  out  from  the  head  a 
fan  of  light  —  the  tail,  consisting  of  one  or  more  luminous 
streaks,  which  vary  in  width  and  length,  are  sometimes 
straight,  sometimes  curved,  but  almost  always  turned  away 
from  the  sun,  forming  the  prolongation  of  a  straight  line 
connecting  the  sun  and  the  comet.  While  telescopic  comets 
are  usually  without  a  tail,  which  causes  them  to  assume  the 
appearance  of  a  more  or  less  irregularly  shaped  nebula, 
possessing  a  nucleus,  an  example  of  which  is  given  in  Do- 
nati's  comet,  as  it  appeared  when  first  seen  on  the  2d  of 
June,  1858,  the  comet  of  July,  1861,  exhibited  two  tails, 
and  the  comet  of  1844  had  even  six. 

Comets  are  transparent  in  every  part,  and  cause  no  refrac- 
tion in  the  light  of  the  stars  seen  through  them.  Bessel  saw 
a  fixed  star  through  Halley's  comet,  and  Struve  one  through 
Biela's  comet,  when  distant  only  a  few  seconds  from  the 
centre  of  the  nucleus,  which  passed  over  the  star  in  both 
instances  without  either  rendering  it  invisible  or  even  per- 
ceptibly fainter ;  from  accurate  measures  taken  at  the  time, 
and  the  calculated  motion  of  the  comet,  it  was  evident  that 
the  position  of  the  star  had  not  been  changed  by  any  refrac- 
tion of  the  light. 

Similar  observations  were  made  with  respect  to  Donati's 
comet  of  1858,  and  the  comet  of  July,  1861.  Close  to  the 
head  of  the  former,  where  the  tail  at  its  commencement  was 
about  54,000  miles  in  thickness,  Arcturus  was  seen  to  shine 
with  undiminished  brightness ;  while  in  both  comets  a 
number  of  fixed  stars  appeared  in  full  brilliancy  through 
even  a  much  thicker  portion  of  the  tail.  The  cornet  of  1828 
possessed  a  nucleus  about  528,000  miles  in  diameter,  and  yet 
Struve  saw  a  star  of  the  eleventh  magnitude  through  it  — 
a  fact  which  seems  to  justify  the  conclusion  of  Babinet, 
drawn  from  his  own  observations,  that  a  comet  "has  no  influ- 
ence upon  the  light  of  a  star,  and  that  stars  of  the  tenth  and 
eleventh  magnitude,  and  some  even  fainter,  may  be  seen 

12 


SCHELLEN.]      SPECTRUM  ANALYSIS  DISCOVERIES.  155 

through  their  greatest  mass,  without  losing  in  the  smallest 
degree  either  their  light  or  their  color. 

The  tail  is  a  prolongation  of  the  coma,  and  is  in  most 
cases  turned  away  from  the  sun,,  whether  the  comet  be 
approaching  or  receding  from  the  sun  in  the  course  of  its 
orbit. 

As  a  comet  approaches  the  sun,  the  tail  regularly  in- 
creases, from  which  it  appears  that  the  sun,  whether  by  the 
action  of  heat  or  other  means,  contributes  essentially  to  the 
formation  of  the  tail,  and  produces  a  separation  of  material 
particles  from  the  head  of  the  comet.  The  length  of  the  tail 
is  rarely  less  than  500,000  miles,  and  in  some  cases  it  extends 
as  far  as  100,000,000  or  150,000,000  miles.  The  breadth  of 
the  tail  of  the  great  comet  of  1811,  at  its  widest  part,  was 
nearly  14,000,000  miles,  the  length  116,000,000;  and  that  of 
the  second  comet  of  the  same  year  even  140,000,000  miles. 
And  yet  the  formation  of  the  tail  takes  place  in  a  very  short 
space  of  time,  often  in  a  few  weeks,  or  even  days. 

The  influence  exercised  on  the  formation  of  the  tail  by  its 
approach  to  the  sun  was  shown  in  the  comet  of  1680,  for  at 
its  perihelion  it  travelled  at  the  rate  of  1,216,800  miles  in  an 
hour,  and,  as  a  consequence,  put  forth  a  tail  in  two  days 
54,000,000  miles  in  length. 

It  is  easily  conceivable  that,  under  such  circumstances,  the 
mass  of  a  comet  must  be  exceedingly  small.  It  is  very 
probable  that  our  earth  actually  passed,  on  the  3oth  of  June, 
1861,  through  part  of  the  tail  of  the  magnificent  comet  called 
the  July  comet,  which  suddenly  appeared  in  the  heavens,  as 
if  by  magic,  on  the  29th  of  June,  and  no  indication  of  such 
a  contact  was  evinced,  beyond  a  peculiar  phosphorescence 
in  the  atmosphere,  which  was  noticed  by  Mr.  Hind,  and  also 
at  the  Liverpool  Observatory.  In  the  same  way,  the  comet 
of  1776  passed  among  the  satellites  of  Jupiter  without  dis- 
turbing their  position  in  the  slightest  degree.  This  was  not 
the  case,  however,  with  the  comet,  for  the  influence  of  the 
planet  was  so  great  on  its  small  mass  as  to  send  it  quite  out 

13 


156  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLEN. 

of  its  course,  into  an  entirely  new  orbit,  which  it  now  ac- 
complishes in  about  twenty  years. 

We  must  now  consider  the  remarkable  phenomenon  of  a 
comet  being  divided  into  two  parts,  each  part  becoming  a 
separate  comet,  and  pursuing  an  orbit  of  its  own.  Such  an 
occurrence  happened  to  Biela's  comet  while  under  observa- 
tion in  the  year  1845.  When  observed  on  the  26th  of  No- 
vember of  that  year,  it  appeared  as  a  faint  nebulous  spot, 
not  perfectly  round,  with  an  increased  density  towards  the 
middle.  On  the  I9th  of  December  it  was  rather  more  elon- 
gated, and  ten  days  later  it  had  become  divided  into  two 
separate,  cloudy  masses  of  equal  dimensions,  each  furnished 
with  a  nucleus  and  tail,  and  for  three  months  one  followed 
the  other  at  a  distance  of  one  tenth,  subsequently  one  fifth, 
of  the  moon's  diameter.  The  pair  made  their  appearance 
again  in  August,  1852,  after  having  travelled  together  in  one 
common  orbit  round  the  sun  for  more  than  six  years  and  a 
half;  but  the  distance  between  them  had  much  increased, 
and  from  154,000  miles,  it  had  now  reached  1,404,000  miles. 
Nor  is  this  all :  in  conformity  with  its  known  period,  the 
return  of  this  comet  was  expected  in  the  year  1859,  anc^ 
again  in  1866,  when  it  must  have  been  visible  from  the 
earth,  as  its  path  crossed  the  earth's  orbit  at  the  place  where 
the  earth  was  on  the  3oth  of  November.  Notwithstanding 
the  most  diligent  search,  however,  the  comet  could  not  be 
found,  and  it  would  seem  that  either,  like  Lexell's  comet, 
it  has  been  drawn  out  of  its  orbit  by  some  member  of  the 
solar  system,  or  else,  as  analogy  suggests,  it  has  ceased 
to  be  a  comet,  and  has  passed  into  some  other  form  of 
existence. 

We  must  enter  a  little  further  than  might  seem  needful  for 
our  purpose  into  the  important  phenomena  observed  in  com- 
ets, partly  by  the  naked  eye,  but  more  especially  by  the  tel- 
escope, in  order  to  obtain  some  ground  for  answering  queries 
as  to  the  physical  nature  of  these  heavenly  bodies,  as  well  as 
to  acquire  a  standard  by  which  to  compare  the  facts  collected 

H 


SCHELLEN.]    SPECTRUM  ANALYSIS  DISCOVERIES.  157 

by  telescopic  observation  with  those  gathered  by  spectrum 
analysis. 

These  questions  are  directed,  in  the  first  place,  to  the  con- 
sideration of  whether  comets,  like  fixed  stars  and  nebulae, 
are  self-luminous,  or  whether,  like  planets,  they  shine  by  the 
reflected  light  of  the  sun ;  in  the  second  place,  to  the  con- 
sideration of  their  material  composition  and  physical  consti- 
tution. That  the  nucleus  of  a  comet  cannot  be  in  itself  a 
dark  and  solid  body,  such  as  the  planets  are,  is  proved  by  its 
great  transparency  ;  but  this  does  not  preclude  the  possibility 
of  its  consisting  of  innumerable  solid  particles,  separated 
one  from  another,  which,  when  illuminated  by  the  sun,  give 
by  the  reflection  of  the  solar  light  the  impression  of  a  homo- 
geneous mass.  It  has  therefore  been  concluded  that  comets 
are  either  composed  of  a  substance  which,  like  gas  in  a  state 
of  extreme  rarefaction,  is  perfectly  transparent,  or  of  small, 
solid  particles,  individually  separated  by  intervening  spaces, 
through  which  the  light  of  a  star  can  pass  without  obstruc- 
tion, and  which,  held  together  by  mutual  attraction,  as  well 
as  by  gravitation  towards  a  central  denser  conglomeration, 
moves  through  space  like  a  cloud  of  dust.  It  is  not  impos- 
sible that  comets  without  a  nucleus  are  masses  of  gas  at  a 
white  heat,  of  similar  constitution  to  the  nebula?,  while  those 
possessing  a  nucleus  are  composed  of  disengaged  solid  par- 
ticles. In  any  case,  the  connection  lately  noticed  by  Schi- 
aparelli  between  comets  and  meteor  showers  seems  to 
necessitate  the  supposition  that  in  many  comets  a  similar 
aggregation  of  particles  exists. 

Donati,  at  Florence,  was  the  first  to  examine  spectroscop- 
ically  the  light  of  comets :  he  compared  the  spectrum  of  the 
comet  I.,  1864,  with  the  spectra  of  metals,  in  which  the  dark 
places  were  wider  than  the  luminous  parts,  and  he  found 
that  the  entire  spectrum  consisted  of  three  bright  lines. 

Tempel's  comet  was  observed  in  January,  1866,  by  Secchi 
and  Huggins,  who  found  that  it  yielded  a  continuous  spec- 
trum exceedingly  faint  at  the  two  ends,  in  which  three  bright 

15 


158  RECREATIONS   IN   POPULAR   SCIENCE.  [SCHELLKN. 

lines  were  seen  by  the  former  observer,  and  only  one  by 
Huggins.  It  appears  from  this,  that  the  nucleus  is  at  least 
-partially  self-luminous,  and  is  composed  of  gas  in  a  lumi- 
nous condition.  On  the  other  hand,  the  continuous  spec- 
trum proves  that  some  of  the  light  is  reflected  sunlight,  for 
it  cannot  be  admitted  that  the  coma  is  formed  of  incandes- 
cent solid  or  liquid  particles. 

The  spectroscope  gives  no  information  as  to  the  nature  or 
condition  of  a  substance  from  which  we  receive  only  re- 
flected light ;  it  is,  however,  probable  that  the  coma  and  tail 
are  of  the  same  substance  as  the  nucleus.  These  observa- 
tions, therefore,  yield  no  further  result  than  that  a  gas  in  a 
state  of  luminosity  is  present  in  the  comet,  but  that,  at  the 
same  time,  either  from  this  gas  or  from  other  portions  of  the 
comet  which  are  non-luminous,  sunlight  is  also  reflected. 

Secchi's  observations  have  been  completely  confirmed  by 
those  of  Huggins.  The  spectrum  of  the  comet  consisted  of 
three  broad,  bright  bands,  which  were  sharply  defined  at 
the  edge  towards  the  red,  but  faded  away  gradually  on  the 
opposite  side. 

It  would  be  premature  to  draw  decisive  results  from  these 
comprehensive  but  as  yet  isolated  observations.  The  spec- 
trum of  the  three  bright  bands  is  derived  unquestionably 
from  the  light  of  the  comet's  nucleus,  and  not  from  that  of 
the  coma,  which  is  far  too  faint  and  ill-defined  to  produce 
such  a  spectrum ;  it  may  therefore  be  assumed  that  the  nu- 
cleus is  self-luminous,  and  that  it  is  very  possibly  composed 
of  glowing  gas  containing  carbon. 

By  collating  these  various  phenomena,  the  conviction  can 
scarcely  be  resisted  that  the  nuclei  of  comets  not  only  emit 
their  own  light,  which  is  that  of  a  glowing  gas,  but  also, 
together  with  the  coma  and  the  tail,  reflect  the  light  of  the 
sun.  There  seems,  therefore,  nothing  to  contradict  the  the- 
ory that  the  mass  of  a  comet  may  be  composed  of  minute 
solid  bodies,  kept  apart  one  from  another  in  the  same  way 
as  the  infinitesimal  particles  forming  a  cloud  of  dust  or 

16 


SCHBLLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  159 

smoke  are  held  loosely  together,  and  that  as  the  comet  ap- 
proaches the  sun,  the  most  easily  fusible  constituents  of 
these  small  bodies  become  wholly  or  partially  vaporized, 
and,  in  a  condition  of  white  heat,  overtake  the  remaining 
solid  particles,  and  surround  the  nucleus  in  a  self-luminous 
cloud  of  glowing  vapor.  Spectrum  analysis  will  not  be  able 
to  afford  any  more  certain  evidence  regarding  the  physical 
nature  of  comets  until  the  appearance  of  a  really  brilliant 
comet,  which  can  be  examined  in  the  various  phases  it  may 
present. 

It  would  lead  us  too  far  from  our  purpose,  were  we  to  de- 
scribe more  minutely  the  extremely  interesting  phenomena 
which  the  telescope  has  revealed  of  the  separation  of  cometic 
matter,  and  the  gradual  formation  of  the  coma  and  tail ;  nor 
can  we  enter  more  fully  here  into  the  causes  of  the  changes 
produced  in  the  form  of  a  comet  by  its  approach  to  the  sun, 
or  to  one  of  the  larger  planets  ;  but  we  cannot  pass  over  the 
extremely  ingenious  hypothesis  brought  forward  by  Pro- 
fessor Tyndall,  before  the  Philosophical  Society  of  Cam- 
bridge, on  the  8th  of  March,  1869.  This  admirable  investi- 
gator had  already  proved,  by  a  series  of  interesting  experi- 
ments, that  concentrated  solar  light,  or  the  electric  light, 
decomposes  the  volatile  vapors  of  many  liquids,  producing 
almost  instantly  a  precipitate  of  cloudy  matter,  in  which 
some  very  peculiar  phenomena  of  light  are  displayed.  The 
quantity  of  vapor  may  be  so  small  as  to  escape  detection, 
but  the  concentrated  light  falling  upon  it  soon  forms  a  blue 
cloud  from  the  moving  atoms  of  vapor  which  now  become 
visible,  and  appear,  according  to  the  nature  of  the  vapor,  in 
a  variety  of  forms,  as  precipitations  of  matter  on  the  beams 
of  light. 

It  is  very  striking,  in  this  experiment,  to  see  the  astonishing 
amount  of  light  that  an  infinitesimal  amount  of  decompos- 
able vapor  is  able  to  reflect.  When  the  electric  light  is 
admitted  into  the  tube,  nothing  is  to  be  seen  for  the  first 
moment ;  but  soon  a  blue  cloud  shows  itself,  which  is 

'7 


160  RECREATIONS   IN   POPULAR   SCIENCE.   [SCHELLEK. 

formed  of  almost  infinitely  small,  particles,  either  of  vapor, 
or,  what  is  more  probable,  of  the  molecules  set  free  by  its 
decomposition,  and  after  some  minutes  the  whole  tube  is 
filled  with  this  blue  color.  The  vaporous  particles  gradually 
augment  in  magnitude,  and  after  some  time  (from  ten  to 
fifteen  minutes)  a  dense  white  cloud  fills  the  tube,  which  dis- 
charges so  great  a  body  of  light  that  it  is  scarcely  conceiv- 
able how  so  small  a  quantity  of  matter  can  possibly  reflect 
so  much  light. 

"  Nothing,"  says  Tyndall,  "  could  more  perfectly  illustrate 
that  *  spiritual  texture'  which  Sir  John  Herschel  ascribes  to 
a  comet,  than  these  actinic  clouds.  Indeed,  the  experiments 
prove  that  matter  of  almost  infinite  tenuity  is  competent  to 
shed  forth  light  far  more  intense  than  that  of  the  tails  of 
comets." 

FALLING  STARS,  METEOR  SHOWERS,  BALLS  OF  FIRE,  AND 
THEIR  SPECTRA. 

Whoever  has  observed  the  heavens  on  a  clear  night  with 
some  amount  of  attention  and  patience,  cannot  fail  to  have 
noticed  the  phenomenon  of  a  falling  star,  one  of  those  well- 
known  fiery  meteors  which  suddenly  blaze  forth  in  any 
quarter  of  the  heavens,  descend  towards  the  earth,  generally 
with  great  rapidity,  in  either  a  vertical  or  slanting  direction, 
and  disappear  after  a  few  seconds  at  a  higher  or  lower  alti- 
tude. As  a  rule,  falling  stars  can  only  be  seen  of  an  evening, 
or  at  night,  owing  to  the  great  brightness  of  daylight ;  but 
many  instances  have  occurred  in  which  their  brilliancy  has 
been  so  great  as  to  render  them  visible  in  the  daytime,  as 
well  when  the  sky  was  overcast  as  when  it  was  perfectly 
cloudless.  It  has  been  calculated  that  the  average  number 
of  these  meteors  passing  through  the  earth's  atmosphere, 
and  sufficiently  bright  to  be  seen  at  night  with  the  naked  eye, 
is  not  less  than  seven  million  and  a  half  during  the  space  of 
twenty-four  hours;  and  this  number  must  be  increased  to 
four  hundred  million,  if  those  be  included  which  a  tele- 

18 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  l6l 

scope  would  reveal.  In  many  nights,  however,  the  number 
of  these  meteors  is  so  great,  that  they  pass  over  the  heavens 
like  flakes  of  snow,  and  for  several  hours  are  too  numerous 
to  be  counted.  Early  in  the  morning  of  the  I2th  of  No- 
vember, 1799,  Humboldt  and  Bonpland  saw  before  sunrise, 
when  on  the  coast  of  Mexico,  thousands  of  meteors  during 
the  space  of  four  hours,  most  of  which  left  a  track  behind 
them  of  from  5°  to  10°  in  length ;  they  mostly  disappeared 
without  any  display  of  sparks,  but  some  seemed  to  burst, 
and  others,  again,  had  a  nucleus  as  bright  as  Jupiter,  which 
emitted  sparks.  On  the  12th  of  November,  1833,  there  fell 
another  shower  of  meteors,  in  which,  according  to  Arago's 
estimation,  two  hundred  and  forty  thousand  passed  over  the 
heavens,  as  seen  from  the  place  of  observation,  in  three 
hours. 

Only  in  very  rare  instances  do  these  fiery  substances  fall 
upon  the  surface  of  the  earth  ;  when  they  do,  they  are  called 
balls  of  fire ;  and  occasionally  they  reach  the  earth  before 
they  are  completely  burnt  out  or  evaporated  ;  they  are  then 
termed  meteoric  stones,  aerolites,  or  meteoric  iron.  They 
are  also  divided  into  accidental  meteors  and  meteoric 
showers,  according  as  to  whether  they  traverse  the  heavens 
in  every  direction  at  random,  or  appear  in  great  numbers 
following  a  common  path,  thus  indicating  that  they  are  parts 
of  a  great  whole. 

It  is  now  generally  received,  and  placed  almost  beyond 
doubt  by  the  recent  observations  of  Schiaparelli,  Le  Verrier, 
Weiss,  and  others,  that  these  meteors,  for  the  most  part 
small,  but  weighing  occasionally  many  tons,  are  fragmentary 
masses,  revolving,  like  the  planets,  round  the  sun,  which  in 
their  course  approach  the  earth,  and,  drawn  by  its  attraction 
into  our  atmosphere,  are  set  on  fire  by  the  heat  generated 
through  the  resistance  offered  by  the  compressed  air. 

The  chemical  analysis  of  those  meteors  which  have  fallen 
to  the  earth  in  a  half-burnt  condition  in  the  form  of  meteoric 
stones,  proves  that  they  are  composed  only  of  terrestrial  ele- 


1 62  RECREATIONS   IN  POPULAR   SCIENCE.    [SCHELLEM. 

merits,  which  present  a  form  and  combination  commonly 
met  with  in  our  planet.  Their  chief  constituent  is  metallic 
iron,  mixed  with  various  silicious  compounds ;  in  combina- 
tion with  iron,  nickel  is  always  found,  and  sometimes  also 
cobalt,  copper,  tin,  and  chromium ;  among  the  silicates, 
olivine  is  especially  worthy  of  remark  as  a  mineral  very 
abundant  in  volcanic  rocks,  as  also  augite.  There  have  also 
been  found  in  the  meteoric  stones  hitherto  examined,  oxygen, 
hydrogen,  sulphur,  phosphorus,  carbon,  aluminium,  magne- 
sium, calcium,  sodium,  potassium,  manganese,  titanium,  lead, 
lithium,  and  strontium. 

The  height  at  which  meteors  appear  is  very  various,  and 
ranges  chiefly  between  the  limits  of  forty-six  and  ninety-two 
miles.  The  mean  may  be  taken  at  sixty-six  miles.  The 
speed  at  which  they  travel  is  also  various,  generally  about 
half  as  fast  again  as  that  of  the  earth's  motion  round  the 
sun,  or  about  twenty-six  miles  in  a  second :  the  maximum 
and  minimum  differ  greatly  from  this  amount,  the  velocity 
of  some  meteors  being  estimated  at  fourteen  miles,  and  that 
of  others  at  one  hundred  and  seven  miles  in  a  second. 

When  a  dark  meteorite  of  this  kind,  having  a  velocity  of 
one  thousand  six  hundred  and  sixty  miles  per  minute,  en- 
counters the  earth,  flying  through  space  at  a  mean  rate  of 
one  thousand  one  hundred  and  forty  miles  per  minute,  and 
when  through  the  earth's  attraction  its  velocity  is  further 
increased  two  hundred  and  thirty  miles  per  minute,  this  body 
meets  with  such  a  degree  of  resistance,  even  in  the  highest 
and  most  rarefied  state  of  our  atmosphere,  that  it  is  impeded 
in  its  course,  and  loses  in  a  very  short  time  a  considerable 
part  of  its  momentum.  By  this  encounter  there  follows  a 
result  common  to  all  bodies  which,  while  in  motion,  sud- 
denly experience  a  check.  When  a  wheel  revolves  very 
rapidly,  the  axletree  or  the  drag  which  is  placed  under  the 
wheel  is  made  red  hot  by  the  friction.  When  a  cannon  ball 
strikes  suddenly  with  great  velocity  against  a  plate  of  iron, 
which  constantly  happens  at  target  practice,  a  spark  is  seen 

20 


SCHELLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  163 

to  flash  from  the  ball,  even  in  daylight ;  under  similar  cir- 
cumstances, a  lead  bullet  becomes  partially  melted.  The 
heat  of  a  body  consists  in  the  vibratory  motion  of  its  small- 
est particles ;  an  increase  of  this  molecular  motion  is  synon- 
ymous with  a  higher  temperature  ;  a  lessening  of  this  vibra- 
tion is  termed  decreasing  heat,  or  the  process  of  cooling. 
Now,  if  a  body  in  motion  —  as,  for  instance,  a  cannon  ball 
—  strike  against  an  iron  plate,  or  a  meteorite  against  the 
earth's  atmosphere,  in  proportion  as  the  motion  of  the  body 
diminishes,  and  the  external  action  of  the  moving  mass  be- 
comes annihilated  by  the  pressure  of  the  opposing  medium 
upon  the  foremost  molecules,  the  vibration  of  these  particles 
increases ;  this  motion  is  immediately  communicated  to  the 
rest  of  the  mass,  and  by  the  acceleration  of  this  vibration 
through  all  the  particles,  the  temperature  of  the  body  is 
raised.  This  phenomenon,  which  always  takes  place  when 
the  motion  of  a  body  is  interrupted,  is  designated  by  the 
expression  the  conversion  of  the  motion  of  the  mass  into 
molecular  action  or  heat  ;  it  is  a  law  without  exception,  that 
where  the  external  motion  of  the  mass  is  diminished,  an 
inner  action  among  its  particles,  or  heat,  is  set  up  in  its  place 
as  an  equivalent,  and  it  may  be  easily  supposed  that  even  in 
the  highest  and  most  rarefied  strata  of  the  earth's  atmos- 
phere, the  velocity  of  the  meteorite  would  be  rapidly  dimin- 
ished by  its  opposing  action,  so  that,  shortly  after  entering 
our  atmosphere,  the  vibration  of  the  inner  particles  would 
become  accelerated  to  such  a  degree  as  to  raise  them  to  a 
white  heat,  when  they  would  either  become  partially  fused, 
or,  if  the  meteorite  were  sufficiently  small,  it  would  be  dissi- 
pated into  vapor,  and  leave  a  luminous  track  behind  it  of 
glowing  gas. 

Haidinger,  in  a  theory  embracing  all  the  phenomena  of 
meteorites,  explains  the  formation  of  a  ball  of  fire  round  the 
meteor  by  supposing  that  the  meteorite,  in  consequence  of 
its  rapid  motion  through  the  atmosphere,  presses  the  air  be- 
fore it  till  it  becomes  luminous.  The  compressed  air  in 

21 


164  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLEN. 

which  the  solid  particles  of  the  surface  of  the  meteorite 
glow  then  rushes  on  all  sides,  but  especially  over  the  surface 
of  the  meteor  behind  it,  where  it  encloses  a  pear-shaped  vac- 
uum which  has  been  left  by  the  meteorite,  and  so  appears  to 
the  observer  as  a  ball  of  fire.  If  several  bodies  enter  the 
earth's  atmosphere  in  this  way  at  the  same  time,  the  largest 
among  them  precedes  the  others,  because  the  air  offers  the 
least  resistance  to  its  proportionately  smallest  surface ;  the 
rest  follow  in  the  track  of  the  first  meteor,  which  is  the  only 
one  surrounded  by  a  ball  of  fire.  When,  by  the  resistance 
of  the  air,  the  motion  of  the  meteor  is  arrested,  it  remains 
for  a  moment  perfectly  still ;  the  ball  of  fire  is  extinguished, 
the  surrounding  air  rushes  suddenly  into  the  vacuum  behind 
the  meteor,  which,  left  solely  to  the  action  of  gravitation, 
falls  vertically  to  the  earth.  The  loud,  detonating  noise 
usually  accompanying  this  phenomenon  finds  an  easy  ex- 
planation in  the  violent  concussion  of  the  air  behind  the 
meteor,  while  the  generally  received  theory  that  the  detonat- 
ing noise  is  the  result  of  an  explosion  or  bursting  of  the 
meteorite,  does  not  meet  with  any  confirmation. 

The  circumstance  that  most  meteors  are  extinguished  be- 
fore reaching  the  earth  seems  to  show  that  their  mass  is  but 
small ;  but  this  is  not  always  the  case.  If  the  distance  of  a 
meteor  from  the  earth  be  ascertained,  as  well  as  its  apparent 
brightness  as  compared  with  that  of  a  planet,  it  is  possible, 
by  comparing  its  luminosity  with  that  of  a  known  quantity 
of  ignited  gas,  to  estimate  the  degree  of  heat  evolved  in  the 
meteor's  combustion.  As  this  heat  originates  from  the  mo- 
tion of  the  meteor  being  impeded  or  interrupted  by  the 
resistance  of  the  air,  and  as  this  motion  or  momentum  is 
exclusively  dependent  on  the  speed  of  the  meteor,  as  well  as 
upon  its  mass,  it  is  possible,  when  the  rate  of  motion  has 
been  ascertained  by  direct  observation,  to  determine  the 
mass.  Professor  Alexander  Herschel  has  calculated,  by  this 
means,  that  those  meteors  of  the  9th  and  loth  of  August, 
1863,  which  equalled  the  brilliancy  of  Venus  and  Jupiter, 

22 


SCHELLEN.]      SPECTRUM  ANALYSIS  DISCOVERIES.  165 

must  have  possessed  a  mass  of  from  five  to  eight  pounds, 
while  those  which  were  only  as  bright  as  stars  of  the  second 
or  third  magnitude  would  not  be  more  than  about  ninety 
grains  in  weight.  As  the  greater  number  of  meteors  are 
less  bright  than  stars  of  the  second  magnitude,  the  faint 
meteors  must  weigh  only  a  few  grains  ;  for  according  to  Pro- 
fessor Herschel's  computation,  the  five  meteors  observed  on 
the  1 2th  of  November,  1865,  some  of  which  surpassed  in 
brilliancy  stars  of  the  first  magnitude,  had  not  an  average 
weight  of  more  than  five  grains  ;  and  Schiaparelli  estimated 
the  weight  of  a  meteor  from  other  phenomena  to  be  about 
fifteen  grains.  The  mass,  however,  of  the  meteoric  stones 
which  fall  to  the  earth  is  considerably  greater,  whether  they 
consist  of  one  single  piece,  such  as  the  celebrated  iron-stone 
discovered  by  Pallas  in  Siberia,  which  weighed  about  two 
thousand  pounds,  and  the  meteorites  of  the  Mexican  desert,* 

*  THE  METEORITES  OF  THE  MEXICAN  DESERT. —With  the  ob- 
ject of  fixing  with  greater  precision  the  geographical  position  of 
the  meteoric  masses  that  have  been  from  time  to  time  met  with  on 
the  Bolson  de  Mapini,  Dr.  J.  Lawrence  Smith  has  communicated  a 
paper  to  the  American  Journal  of  Science  for  November,  1871,  335. 
There  were  already  known  the  Cohahuila  meteorite  of  1854  (0>  tne 
Cohahuila  meteorite  of  1868  (2),  the  Chihuahua  iron  of  1854  (3), 
still  at  the  Hacienda  de  Conception,  and  weighing  about  four  thou- 
sand pounds,  and  the  Tucson  iron  (4),  found  in  1854  on  the  north 
of  the  Rio  Grande,  and  having  the  form  of  a  ring.  This  mass 
weighs  fr6m  two  to  three  thousand  pounds.  A  fifth  mass  (5)  has 
since  been  heard  of  on  the  western  border  of  the  Mexican  desert, 
that  has  received  the  name  of  the  San  Gregario  meteoric  iron.  It 
measures  six  feet  six  inches  in  length,  is  five  feet  six  inches  high, 
and  four  feet  thick  at  the  base.  On  one  part  of  its  surface,  1821  has 
been  cut  with  a  chisel.  Its  weight  is  calculated  to  be  about  five 
tons.  An  examination  of  a  fragment  showed  it  to  be  one  of  the 
softer  meteoric  irons.  It  has  a  specific  gravity  of  7.84,  and  consists 
of  ninety-five  per  cent,  of  iron,  and  five  per  cent,  of  nickel,  inclu- 
sive of  a  little  cobalt.  Still  more  recently,  news  has  arrived  of  the 
discovery,  in  the  central  portion  of  the  desert,  of  a  huge  meteorite 
(6),  larger  than  any  yet  found  in  this  locality.  Dr.  Lawrence 
Smith's  paper  is  illustrated  with  a  little  map,  indicating  their  rel- 

23 


166  RECREATIONS  IN  POPULAR  SCIENCE.     [SCHELLE*. 

or  of  a  cloud  composed  of  many  small  bodies,  which  pen- 
etrate the  earth's  atmosphere  in  parallel  paths,  and  which, 
from  a  simultaneous  ignition  and  descent  upon  the  earth, 
present  the  appearance  of  a  large  meteor  bursting  into  sev- 
eral smaller  pieces.  Such  a  shower  of  stones,  accompanied 
by  a  bright  light  and  loud  explosion,  occurred  at  L'Aigle,  in 
Normandy,  on  the  26th  of  April,  1803,  when  the  number  of 
stones  found  in  a  space  of  fourteen  square  miles  exceeded 
two  thousand.  In  the  meteoric  shower  that  fell  at  Kiiya- 
hinga,  in  Hungary,  on  the  9th  of  June,  1866,  the  principal 
stone  weighed  about  eight  hundred  pounds,  and  was  accom- 
panied by  about  a  thousand  smaller  stones,  which  were 
strewed  over  an  area  of  nine  miles  in  length  by  three  and 
one  quarter  broad. 

It  must  not  be  supposed,  however,  that  the  density  of  such 
a  cosmieal  cloud  is  as  great,  when  out  of  the  reach  of  the 
attraction  of  the  sun  and  the  earth,  as  when  its  constituents 
fall  upon  the  earth's  surface.  Schiaparelli  calculates,  from 
the  number  of  meteors  observed  yearly  in  the  month  of  Au- 
gust, that  the  distance  between  any  two  must  amount,  on  the 
average,  to  four  hundred  and  sixty  miles.  As  the  cosmieal 
clouds  which  produce  the  meteors  approach  the  sun  in  their 
wanderings  from  the  far-off  regions  of  space,  they  increase 
in  density  some  million  times  ;  therefore  the  distance  between 
any  two  meteors,  only  a  few  grains  in  weight,  before  the 
cloud  begins  to  be  condensed,  may  be  upwards  of  forty 
thousand  miles. 

The  most  striking  example  of  such  a  cosmieal  cloud, 
composed  of  small  bodies  loosely  hung  together,  and  existing 

ative  positions.  He  believes  they  are  the  result  of  two  falls.  The 
Tucson  iron  has  characters  that  distinguish  it  from  the  remaining 
five.  The  latter,  he  considers,  fell  at  an  epoch  probably  far  remote, 
moving  from  north-west  to  south-west  during  their  descent.  I  and 
2  fell  first,  85  miles  apart.  The  distances  between  the  larger  masses 
are,  —  from  2  to  6,  135  miles;  from  6  to  5,  165  miles;  and  from  5  to 
3,  about  90  miles 

24 


SCHELLEN.J     SPECTRUM  ANALYSIS  DISCOVERIES.  167 

with  hardly  any  connection  one  with  another,  is  exhibited  in 
the  meteoric  showers  occurring  periodically  in  August  and 
November.  It  is  an  ascertained  fact  that,  on  certain  nights 
in  the  year,  the  number  of  meteors  is  extraordinarily  great, 
and  that  at  these  times  they  shoot  out  from  certain  fixed 
points  in  the  heavens.  The  shower  of  meteors  which  hap- 
pens every  year  on  the  night  of  the  loth  of  August,  pro- 
ceeding from  the  constellation  of  Perseus,  is  mentioned  in 
many  old  writings.  The  shower  of  the  I2th  and  I3th  of 
November  occurs  periodically  every  thirty-three  years,  for 
three  years  in  succession,  with  diminishing  numbers ;  it  was 
this  shower  that  Alexander  Von  Humboldt  and  Bonpland 
observed  on  the  I2th  of  November,  1799,  as  a  real  rain  of 
fire.  It  recurred  on  the  I2th  of  November,  1833,  in  such 
force  that  Arago  compared  it  to  a  fall  of  snow,  and  was  lately 
observed  again  in  its  customary  splendor  in  North  America, 
on  the  1 4th  of  November,  1867.  Besides  these  two  prin- 
cipal showers,  there  are  almost  a  hundred  others,  recurring 
at  regular  intervals ;  each  of  these  is  a  cosmical  cloud,  com- 
posed of  small,  dark  bodies,  very  loosely  held  together,  like 
the  particles  of  a  sand  cloud,  which  circulate  round  the  sun 
in  one  common  orbit;  The  orbits  of  these  meteor  streams 
are  very  diverse  ;  they  do  not  lie  approximately  in  one  plane, 
like  those  of  the  planets,  but  cross  the  plane  of  the  earth's 
orbit  at  widely  different  angles.  The  motion  of  the  indi- 
vidual meteors  ensues  in  the  same  direction  in  one  and  the 
same  orbit ;  but  this  direction  is  in  some  orbits  in  conformity 
with  that  of  the  earth  and  planets,  while  in  others  it  is  in  the 
reverse  order. 

The  earth,  in  its  revolution  round  the  sun,  occupies  every 
day  a  different  place  in  the  universe;  if,  therefore,  a  mete- 
oric shower  pass  through  our  atmosphere  at  regular  inter- 
vals, there  must  be  at  the  place  where  the  earth  is  at  that 
time  an  accumulation  of  these  small  cosmical  bodies,  which,, 
attracted  by  the  earth,  penetrate  its  atmosphere,  are  ignited! 
by  the  resistance  of  the  air,  and  become  visible  as  falling 

25 


1 68  RECREATIONS   IN   POPULAR   SCIENCE. 

stars.  A  cosmical  cloud,  however,  cannot  remain  at  a  fixed 
spot  in  our  solar  system,  but  must  circulate  round  the  sun  as 
planets  and  comets  do  ;  whence  it  follows  that  the  path  of  a 
periodic  shower  intersects  the  earth's  orbit,  and  the  earth 
must  either  be  passing  through  the  cloud,  or  else  very  near 
to  it,  when  the  meteors  are  visible  to  us. 

The  meteor  shower  of  the  loth  of  August,  the  radiant 
point  of  which  is  situated  in  the  constellation  of  Perseus, 
takes  place  nearly  every  year,  with  varying  splendor ;  we 
may  therefore  conclude  that  the  small  meteors  composing 
this  group  form  a  ring  round  the  sun,  and  the  earth  evqry 
loth  of  August  is  at  the  spot  where  this  ring  intersects  our 
orbit ;  also  that  the  ring  of  meteors  is  not  equally  dense  in 
all  parts:  here  and  there  these  small  bodies  must  be  very 
thinly  scattered,  and  in  some  places  even  altogether  wanting. 

Fig.  XIX.  shows  a  very  small  part  of  the  elliptic  orbit 
which  this  meteoric  mass  describes  round  the  sun  S.  The 
earth  encounters  this  orbit  on  the  loth  of  August,  and  goes 
straight  through  the  ring  of  meteors.  The  dots  along  the 
ring  indicate  the  small,  dark  meteors  which  ignite  in  our 
atmosphere,  and  are  visible  as  shooting  stars.  The  line  m  is 
the  .line  of  intersection  of  the  earth's  orbit  and  that  of  the 
meteors.;  the  line  P  S  shows  the  direction  of  the  major  axis 
of  .their  orbit.  This  axis  is  fifty  times 'greater  than  the  mean 
diameter  of , the  earth's  orbit;  the  orbit  of  the  meteors  is  in- 
clined to  that  of  the  earth  at  an  angle  of  64°  3',  and  their 
motion  is  retrograde,  or  contrary  to  that  of  the  earth. 

The  November  shower  is  not  observed  to  take  place  every 
year  on  the  I2th  or  I3th  of  that  month,  but  it  is  found  that 
every  thirty-three  years  an  extraordinary  shower  occurs  on 
those  days,  proceeding  from  a  point  in  the  constellation  of 
Leo.  The  meteors  composing  this  shower,  unlike  the  Au- 
gust one,  are  not  distributed  along  the  whole  course  of  their 
orbit,  so  as  to  form  a  ring  entirely  filled  with  meteoric  par- 
ticles, but  constitute  a  dense  cloud,  of  an  elongated  form, 
which  completes  its  revolution  round  the  sun  in  thirty-three 

26 


SCHBLLEN.]     SPECTRUM  ANALYSIS  DISCOVERIES.  169 

years,  and  crosses  the  earth's  path  at  that  point  where  the 
earth  is  every  I3th  of  November. 

When  the  November  shower  reappears  after  the  lapse  of 

FIG.  XIX. 


ORBIT    OF    THE    METEOR    SHOWER    OF   THE    10th    OF    AUGUST. 

thirty-three  years,  the  phenomenon  is  repeated  during  the 
two  following  years  on  the  I3th  of  that  month,  but  with 
diminished  splendor ;  the  meteors,  therefore,  extend  so  far 

27 


1 70  RECREATIONS    IN    POPULAR   SCIENCE.   [SCHELLKN. 

along  the  orbit  as  to  require  three  years  before  they  have  all 
crossed  the  earth's  path  at  the  place  of  intersection ;  they 
are,  besides,  unequally  distributed,  the  preceding  part  being 
much  the  most  dense. 

A  very  small  part  of  the  elliptic  orbit,  and  the  distribution 
of  the  meteors  during  the  November  shower,  is  represented 
in  Fig.  XX.  As  shown  in  the  drawing,  this  orbit  intersects 
that  of  the  earth  at  the  place  where  the  earth  is  about  the 
I4th  of  November,  and  the  motion  of  the  meteors,  which 
occupy  only  a  small  part  of  their  orbit,  and  are  very  un- 
equally distributed,  is  retrograde,  or  contrary  to  that  of  the 
earth.  The  inclination  of  this  orbit  to  that  of  the  earth  is 
only  17°  44' ;  its  major  axis  is  about  ten  and  one  third  times 
greater  than  the  diameter  of  the  earth's  orbit,  and  the  period 
of  revolution  for  the  densest  part  of  the  meteorites  round 
the  sun  S  is  thirty-three  years  three  months. 

From  all  we  have  now  learned  concerning  the  nature  and 
constitution  of  comets,  nebulas,  cosmical  clouds,  and  mete- 
oric swarms,  an  unmistakable  resemblance  will  be  remarked 
among  these  different  forms  in  space.  The  affinity  between 
comets  and  meteors  had  been  already  recognized  by  Chladni, 
but  Schiaparelli,  of  Milan,  was  the  first  to  take  account  of 
all  the  phenomena  exhibited  by  these  mysterious  heavenly 
bodies,  and  with  wonderful  acuteness  to  treat  successfully 
the  mass  of  observations  and  calculations  which  had  been 
contributed  during  the  course  of  the  last  few  years  by  Oppol- 
zer,  Peters,  Bruhns,  Heis,  Le  Verrier,  and  other  observers. 
He  not  only  shows  that  the  orbits  of  meteors  are  quite  coin- 
cident with  those  of  comets,  and  that  the  same  object  may 
appear  to  us  at  one  time  as  a  comet  and  at  another  as  a 
shower  of  meteors,  but  he  proves  also,  by  a  highly  elegant 
mathematical  calculation,  that  the  scattered  cosmical  masses 
known  to  us  by  the  name  of  nebulae  would,  if  in  their 
journey  through  the  universe  they  were  to  come  within  the 
powerful  attraction  of  our  sun,  be  formed  into  comets,  and 
these  again  into  meteoric  showers. 

28 


SCHBLLBN.]     SPECTRUM  ANALYSIS   DISCOVERIES.  171 

The  following  is  a  short  statement  of  Schiaparelli's  theory. 
Nebulae  are  composed  of  cosmical  matter,  in  which,  as  yet, 
there  is  no  central  point  of  concentration,  and  which  has  not 
become  sufficiently  dense  to  form  a  celestial  body,'  in  the 
ordinary  sense  of  the  term.  The  diffuse  substance  of  these 
cosmical  clouds  is  very  loosely  hung  together;  its  particles 
are  widely  separated,  thus  constituting  masses  of  enormous 
extent,  some  of  which  have  taken  a  regular  form,  and  some 
not.  As  these  nebulous  clouds  may  be  supposed  to  have, 
like  our  sun,  a  motion  in -space*  it  will  sometimes  happen 
that  such  a  cloud  comes  within  reach  of  the  power  of  attrac- 
tion of  our  sun.  The  attraction  acts  more  powerfully  on 
the  preceding  part  of  the  nebula  than  on  the  further  and  fol- 
lowing portion;  and  the  nebula,  while  still  at  a  great  dis- 
tance, begins  to  lose  its  original  spherical  form,  and  becomes 
considerably  elongated.  Other  portions  of  the  nebulous 
mass  follow  continuously  the  preceding  part,  until  the  sphere 
is  converted  into  a  long  cylinder^  the  foremost  part  of  which, 
that  towards  the  sun,  is  denser  and  more  pointed  than  the 
following •  part,  which  retains  a  portion  of  its  original 
breadth.  As  it  nears  the  sun,  this  transformation  of  the 
nebulous  cloud  becomes  more  complete :  illuminated  by  the 
sun,  the  preceding  part  appears  to  us  as  a  dense  nucleus,  and 
the  following  part,  turned  away  from  the  sun,  as  a  long  tail, 
curved  in  consequence  of  the  lateral  motion  preserved  by  the 
nebula  during  its  progress.  Out  of  the  original  spherical 
nebula,  quite  unconnected  with  our  solar  system,  a  comet 
has  been  formed,  which,  in  its  altered  condition,  will  either 
pass  through  our  system,  to  wander  again  in  space,  or  else 
remain  as  a  permanent  member  of  our  planetary  system. 
The  form  of  the  orbit  in  which  it  moves  depends  on  the 
original  speed  of  the  cloud,  its  distance  from  the  sun,  and 
the  direction  of  its  motion,  and  thus  its  path  may  be  ellipti- 
cal, hyperbolical,  or  parabolical;  in  the  last  two  cases,  the 
comet  appears  only  once  in  our  system,  and  then  returns  to 
wander  in  the  realms  of  space  ;  in  the  former  case,  it  abides 

29 


I72  RECREATIONS    IN    POPULAR    SCIENCE.    [SCHELLEN. 

with  us,  and  accomplishes  its  course  round  the  sun,  like  the 
planets,  in  a  certain  fixed  period  of  years.  From  this  it  is 
evident  that  the  orbits  of  comets  may  occur  at  every  possible 
angle  to  that  of  the  earth,  and  that  their  motion  will  be 
sometimes  progressive  and  sometimes  retrograde. 

The  history  of  the  cosmical  cloud  does  not,  however,  end 
with  its  transformation  into  a  comet.  Schiaparelli  shows 
in  a  striking  manner  that,  as  a  comet  is  not  a  solid  mass,  but 
consists  of  particles  each  possessing  an  independent  motion, 
the  head  or  nucleus  nearer  the  sun  must  necessarily  complete 
its  orbit  in  less  time  than  the  more  distant  portions  of  the 
tail.  The  tail  will  therefore  lag  behind  the  nucleus  in  the 
course  of  the  comet's  revolution,  and  the  comet,  becoming 
more  and  more  elongated,  will  at  last  be  either  partially  or 
entirely  resolved  into  a  ring  of  meteors.  In  this  way,  the 
whole  path  of  the  comet  becomes  strewn  with  portions  of 
its  mass,  —  with  those  small,  dark,  meteoric  bodies  which, 
when  penetrating  the  earth's  atmosphere,  become  luminous, 
and  appear  as  falling  stars.  Instead  of  the  comet,  there  now 
revolves  round  the  sun  a  broad  ring  of  meteoric  stones, 
which  occasion  the  phenomena  we  every  year  observe  as  the 
August  meteors.  Whether  this  ring  be  continuous,  and  the 
meteoric  masses  strewn  along  the  whole  course  of  the  path 
of  the  original  comet,  or  whether  the  individual  meteors,  as 
in  the  November  shower,  have  not  filled  up  entirely  the 
whole  orbit,  but  are  still  partially  in  the  form  of  a  comet,  is 
in  the  transformation  of  a  cosmical  cloud  through  the  influ- 
ence of  the  sun  only  a  question  of  time  ;  in  course  of  years, 
the  matter  composing  a  comet  which  describes  an  orbit 
round  the  sun  must  be  dispersed  over  its  whole  path  ;  if  the 
original  orbit  be  elliptical,  an  elliptic  ring  of  meteors  will 
gradually  be  formed  from  the  substance  of  the  comet,  of 
the  same  size  and  form  as  the  original  orbit. 

Schiaparelli  has  in  fact  discovered  so  close  a  resemblance 
between  the  path  of  the  August  meteors  and  that  of  the 
comet  of  1862,  No.  III.,  that  there  cannot  be  any  doubt  as 

30 


SCHKLLEN.]      SPECTRUM  ANALYSIS  DISCOVERIES.  173 

to  their  complete  identity.  The  meteors  to  which  we  owe 
the  annual  display  of  falling  stars  on  the  loth  of  August 
are  not  distributed  equally  along  the  whole  course  of  their 
orbit ;  it  is  still  possible  to  distinguish  the  agglomeration  of 
meteoric  particles  which  originally  formed  the  cometary 
nucleus  from  the  other  less  dense  parts  of  the  comet ;  thus 
in  the  year  1862,  the  denser  portion  of  this  ring  of  meteors, 
through  which  the  earth  passes  annually  on  the  loth  of  Au- 
gust, and  which  causes  the  display  of  falling  stars,  was  seen 
in  the  form  of  a  comet,  with  head  and  tail  as  the  densest 
parts,  approached  the  sun  and  earth  in  the  course  of  that 
month.  Oppolzer,  of  Vienna,  calculated  with  great  accu- 
racy the  orbit  of  this  comet,  which  was  visible  to  the  naked 
eye.  Schiaparelli  had  previously  calculated  the  orbit  of  the 
meteoric  ring,  to  which  the  shooting  stars  of  the  loth  of 
August  belong  before  they  are  drawn  into  the  earth's  atmos- 
phere. The  almost  perfect  identity  of  the  two  orbits  justi- 
fies Schiaparelli  in  the  bold  assertion  that  the  comet  of  '1862, 
No.  III.,  is  no  other  than  the  remains  of  the  comet  out  of 
'which  the  meteoric  ring  of  the  ivth  of  August  has  been 
formed  in  the  course  of  time.  The  difference  between  the 
comet's  nucleus  and  its  tail,  that  has  now  been  formed  into  a 
ring,  consists  in  that  while  the  denser  meteoric  mass  forming 
the  head  approaches  so  near  the  earth  once  in  every  hundred 
and  twenty  years  as  to  be  visible  in  the  reflected  light  of  the 
sun,  the  more  widely  scattered  portion  of  the  tail  composing 
the  ring  remains  invisible,  even  though  the  earth  passes 
through  it  annually  on  the  loth  of  August.  Only  fragments 
of  this  ring,  composed  of  dark  meteoric  particles,  become 
visible  as  shooting  stars  when  they  penetrate  our  atmosphere 
by  the  attraction  of  the  earth,  and  ignite  by  the  compression 
of  the  air. 

A  cloud  of  meteors  of  such  a  character  can  naturally  only 
be  observed  as  a  meteor  shower  when  in  the  nodes  of  its 
orbit,  —  that  is  to  say,  in  those  points  where  it  crosses  the 
earth's  orbit,  —  and  then  only  when  the  earth  is  also  there  at 


174  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLEN. 

the  same  time,  so  that  the  meteors  pass  through  our  atmos- 
phere.    The  nebula  coming  within  the  sphere  of  attraction 

FIG.  XX. 


Orbits  of  the  August  and  November  Meteor  Showers. 
(Orbits  of  Comets  III.,  1862,  and  I.,  1866.) 

of  our  solar  system,  would,  at  its  nearest  approach  to  the 
sun  (perihelion),  and  in  .the  neighboring  portions  of  its  orbit, 

32 


SCHELLEN.]    SPECTRUM  ANALYSIS  DISCOVERIES.  175 

appear  as  a  comet,  and  when  it  grazed  the  earth's  atmos- 
phere would  be  seen  as  a  shower  of  meteors. 

Calculation  shows  that  this  ring  of  meteors  is  about  ten 
thousand  nine  hundred  and  forty-eight  millions  of  miles  in 
its  greatest  diameter.  As  the  meteoric  shower  of  the  loth 
of  August  lasts  about  six  hours,  and  the  earth  travels  at  the 
rate  of  eighteen  miles  in  a  second,  it  follows  that  the  breadth 
of  this  ring,  at  the  place  where  the  earth  crosses  it,  is  four 
million  forty-three  thousand  five  hundred  and  twenty  miles. 
In  Fig.  XX.,  A  B  represents  a  portion  of  the  orbit  of  the 
comet  of  1862,  No.  III.,  which  is  identical  with  that  (Fig. 
XIX.)  of  the  August  shower. 

The  calculations  of  Schiaparelli,  Oppolzer,  Peters,  and 
Le  Verrier  have  also  discovered  the  comet  producing  the 
meteors  of  the  November  shower,  and  have  found  it  in  the 
small  comet  of  1866,  No.  I.,  first  observed  by  Tempel,  of 
Marseilles.  Its  transformation  into  a  ring  of  meteors  has 
not  proceeded  nearly  so  far  as  that  of  the  comet  of  1862, 
No.  III.  Its  existence  is  of  a  much  more  recent  date ;  and 
therefore  the  dispersion  of  the  meteoric  particles  along  the 
orbit,  and  the  consequent  formation  of  the  ring,  is  but 
slightly  developed. 

According  to  Le  Verrier,  a  cosmical  nebulous  cloud  en- 
tered our  system  in  January  126,  and  passed  so  near  the 
planet  Uranus  as  to  be  brought  by  its  attraction  into  an  el- 
liptic orbit  round  the  sun.  This  orbit  is  the  same  as  that  of 
the  comet  discovered  by  Tempel,  and  calculated  by  Oppol- 
zer, and  is  identical  with  that  in  which  the  November  group 
of  meteors  make  their  revolution. 

Since  that  time,  this  cosmical  cloud,  in  the  form  of  a 
comet,  has  completed  fifty-two  revolutions  round  the  sun, 
without  its  existence  being  otherwise  made  known  than  by 
the  loss  of  an  immense  number  of  its  components,  in  the 
form  of  shooting  stars,  as  it  crossed  the  earth's  path  in  each 
revolution,  or  in  the  month  of  November  in  every  thirty- 
three  years.  It  was  only  in  its  last  revolution,  in  the  year 

33 


176  RECREATIONS  IN  POPULAR  SCIENCE.    [SCHELLW. 

1866,  that  this  meteoric  cloud,  now  forming  part  of  our  solar 
system,  was  first  seen  as  a  comet. 

The  orbit  of  this  comet  is  much  smaller  than  that  of  the 
August  meteors,  extending  at  the  aphelion  as  far  as  the  orbit 
of  Uranus,  while  the  perihelion  is  nearly  as  far  from  the  sun 
as  our  earth.  The  comet  completes  its  revolution  in  about 
thirty-three  years  and  three  months,  and  encounters  the 
earth's  orbit  as  it  is  approaching  the  sun  towards  the  end  of 
September.  It  is  followed  by  a  large  group  of  small  mete- 
oric bodies,  which  form  a  very  broad  and  long  tail,  througli 
which  the  earth  passes  on  the  I3th  of  November.  Those 
particles  which  come  in  contact  with  the  earth,  or  approach 
so  near  as  to  be  attracted  into  its  atmosphere,  become  ignited, 
and  appear  as  falling  stars.  As  the  earth  encounters  the 
comet's  tail,  or  meteoric  shower,  for  three  successive  years 
at  the  same  place,  we  must  conclude  the  comet's  track  to 
have  the  enormous  length  of  seventeen  hundred  and  seventy- 
two  millions  of  miles.  In  Fig.  XX.,  C  D  represents  a  por- 
tion of  the  orbit  of  this  comet,  which  is  identical  with  the 
orbit  of  the  November  meteors. 

By  the  side  of  these  important  conclusions,  which  the  ob- 
servation and  acuteness  of  modern  astronomers  have  been 
able  to  make  concerning  the  nature  and  mutual  connection 
of  nebulae,  comets,  meteors,  and  balls  of  fire,  the  results  of 
spectrum  analysis,  as  applied  to  meteors,  will  seem  to  be 
exceedingly  scant  This  is  easy  to  understand  when  we 
reflect  how  rapidly  these  fiery  meteors  rush  through  our  at- 
mosphere, and  how  difficult  it  is  to  lay  hold  of  them  with 
the  spectroscope  during  their  instantaneous  apparition.  Be- 
fore the  instrument  can  be  directed  to  a  meteor  or  ball  of 
fire,  and  the  focus  adjusted,  the  object  has  disappeared  from 
view.  The  application,  therefore,  of  spectrum  analysis  to 
these  fleeting  visitors  is  left  almost  entirely  to  chance,  and  is 
mainly  confined  to  those  nights  in  which  yearly,  or  at  certain 
known  periods,  an  extraordinary  shower  of  falling  stars  is 
expected  to  occur. 

34 


SCHELLEN.]      SPECTRUM  ANALYSIS  DISCOVERIES.  177 

The  principal  result  of  the  investigations  thus  far  made  is 
confined,  therefore,  to  the  establishment  of  the  fact  that  me- 
teors consist  of  incandescent  solid  bodies,  and  that  a  differ- 
ence is  discernible  in  the  chemical  composition  of  the  August 
and  November  meteoric  showers. 

The  November  shower  of  1868  was  observed  by  Secchi. 
Among  the  numerous  meteors  that  left  a  train  of  light  be- 
hind them,  was  one,  the  track  of  which  lasted  fifteen  min- 
utes, and  was  at  first  sufficiently  bright  to  allow  of  examina- 
tion by  a  prism.  Secchi  found  the  spectrum  to  be  discon- 
tinuous, and  the  principal  bright  bands  and  lines  were  red, 
yellow,  green,  and  blue.  Besides  this  observation,  Secchi 
was  so  fortunate  as  to  see  two  meteors  in  the  spectroscope : 
the  magnesium  line  appeared  with  great  distinctness,  besides 
which  some  lines  were  also  seen  in  the  red. 

On  account  of  the  great  difficulty  of  observing  meteors 
with  a  narrow  setting  of  the  slit,  ordinary  spectroscopes  are 
not  suited  to  this  purpose.  The  only  resource,  therefore,  is 
to  substitute  a  cylindrical  lens  for  the  slit.  There  can  be  no 
doubt,  however,  that  an  apparatus  will  be  invented  which 
will  be  employed  in  future  with  great  success  in  the  investi- 
gation of  meteors  by  means  of  spectrum  analysis. 

35 


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